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The Definitive¹ Guide to Dark Mode and Bootstrap 4

A proof of concept / Long-form white paper [dark paper]

This body of work pertains to Bootstrap 4. If you're after the same work for Bootstrap 5 please visit the vinorodrigues/bootstrap-dark-5 repo.

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About me & The history that led here

Firstly, I'd like to state that I'm not a professional developer - I was, from about '88 to '05, but that's not what I do now. I'm an Electrical Engineer and Businessman who manages data centers and their staff, and design UPS, HVAC and power systems ... that's my day job. But I still love code. I code as a hobby and everything I've coded since 2005 I've open-source... somewhere. I'm also not a "graphics" or "artistic" person - I've no education in design, UI or UX - but I understand it's basic concepts. I'm also not a long form writer... I'm dyslexic and have a relatively low verbal-linguistic Intelligence - so these ramblings may be hard to digest. My apologies.

None of the content here is original; It's based on logical combination of ideas from many great technology content producers, and I've tried to reference these when I can. There is also a lot of personal opinions and my own ideas of code philosophy and design positions - these I will mark with an asterisk (¹). After managing developers for many years, I can safely¹ say that if you put 30 developers in a room and ask them to code something, you'd get 30 different ways to get to the same result. (Tom Scott has a brilliant¹ conversation about this in his video about the FizzBuzz Test.)

That said; my opinions and code path here is just one way to achieve what I though needed to be proved, namely that:

I can't remember when I first came across the concept of "dark mode"; must have been sometime in late 2018 when I'd installed Safari Technology Preview and read some reference to the prefers-color-scheme (still in draft with the W3C). At the time I was attempting to write a (now abandoned) WordPress theme based on Bootstrap 4 after having written a Bootstrap 2 based WP theme a few years earlier, and a a bit later a printing plugin for Bootstrap 3. I remember thinking that I would love to not only release a highly configurable WordPress theme, but one that supported this wonderful css-dark-mode. I can vaguely recall looking at the code and deciding "Yea, not doable¹!" and started doing some research on the topic. But at the time there wasn't much. I did however come across issue #27514 in the Bootstrap GitHub repository, and also saw @mdo) close it off. My thoughts were that if the creator of Bootstrap was not keen on this then it would never happen¹ - so I reluctantly gave up on WP-Bootstrap-4 and moved on to other hobbies.

Time passed - I fell out of love with Bootstrap and took on a new mistress¹, Foundation 6. Spent a lot of time with it, but it is... different... lighter, easier to use because it is simpler, merged awesomely with "your own scss"; but it also doesn't work so well with out-of-the-box inclusion into existing sites - especially if you want to theme it up a bit, and it's not as flexible. Bootstrap has Thomas Park's (@thomaspark) Bootswatch (Why the hell is he not a contributor?¹), that he's been maintaining for over 6 years now. But Foundation has nothing - yes Justin Mahar had started one called Foundswatch in 2018, but he archived that and relinquished the domain name. The funny thing about Justin's work is that I didn't know of it until after I had created my own variant of Foundswatch, but I digress. The important takeaway of that side-story is that whilst I was working on it, I explored deeply the topic of Dark Mode again. I realized that Bootswatch Flatly and Bootswatch Darkly are only different in color. I also came across Thomas Steiner's (@tomayac) awesome article "prefers-color-scheme: Hello darkness, my old friend" and realized that I could offer a dark-mode option to Foundswatch users. And so, for Foundswatch, I created a foundation-dark theme that was usable as a two-color scheme CSS (albeit with 2 CSS files) for Foundation 6 and wrote a how to in the help page.

Sadly, I was not satisfied - I wanted a one CSS files solution - and promptly started to modify my own fork of Foundation for Sites. OMG! It was very near impossible! (More on why later, but needless to say I gave up on that). Then one recent day I get a notification on my RSS reader that Bootstrap 4.5.0 was out ... "hello my old love" I thought and did the pulling and reading and the revisiting of issues pertaining to "dark mode". My old friend #27514 was now active, and then there was a bunch of them including:

But nowhere did I see an attempt at creating a true dark mode - in the core. I did notice my old friend #27514 was marked for V6. V6!!! I can't wait that long¹!

So, I set out to prove that one can achieve true dark-mode support.

The (General) Philosophy¹ of Dark Mode

Before I set out to discuss how I went about coding this I thought it prudent to explore why dark mode is even a thing. I could not find a definitive guide on why dark mode. The history seems assumed - The UX Collective article "The past, present, and future of Dark Mode" that covers the origins from CRT screens - but goes on to reference experiments and studies done with light-on-dark vs. dark-on-light and how a bunch of them showed that darker UI was "more productive". Wired's Will Bedingfield wrote in "How dark mode took over our screens" how companies like Apple, Microsoft, Google and Twitter are all in on the dark mode train. Chris Taylor wrote in Mashable "Why 2019 was the year of Dark Mode" how the push for dark mode has become more prominent in recent times (late 2019, early 2020). There are also umpteenth articles on the benefits of dark mode, like Reeno Koemets in "The Benefits of Dark Mode: Why should you turn off the lights?", and even some against it, like Adamya Sharma's "Love dark mode? Here's why you may still want to avoid it", but the general consensus is that if you're not offering your websites and web-apps in dark mode that you're literally standing out - in a negative way.

So, it thus confounds me, as to why the Bootstrap Authors are not looking at this with more haste and urgency.

The Philosophy of this Proof of Concept

I mentioned before @Carl-Hugo's boostrap-dark theme already produces a dark variant of Bootstrap and, combined with the approach Thomas Steiner (@tomayac) suggests, one can certainly achieve dark mode today - though I've not seen those two bodies of work linked.

But the 2-CSS file approach - though very legitimate and very usable - has a flaw if not used correctly. Support for older browsers - specifically the fact that, in older browsers like IE11, they will load both CSS and neither will render. You'll need additional JavaScript code to inject a non-media filtered CSS ... not a bad thing, but it makes drop-in replacement of existing sites using Bootstrap needing some level of code modification - some easy, some harder (think WordPress themes).

On the plus side this approach is about the only one that gives you true flexibility. You can for example use 3rd party styling (like Bootswatch Flatly and Darkly) to generate the same effect. This is way more flexible.

In my opinion what's ideal is a single CSS Bootstrap variant¹ that does dark mode for browsers that support it, but also works in all supported browsers.

There is a bunch of conversation from the core authors around support for dark mode based on CSS variables and that their only concern was IE11 and that by the time they get to dark mode then they'd drop support for IE11. All fine and dandy, but it's not only IE11 that does not support dark mode, as in the prefers-color-scheme media query, and CSS variables. There is also a bunch of older mobile devices still in use that cannot be upgraded, for example older iPad that cannot upgrade further than iOS 12.4 and Safari 12.1. One cannot forget that only 58% of used browsers (based on the sum of top 10 browsers supporting it, from "Browser & Platform Market Share April 2020") support dark mode today - so it makes sense that the Authors want to wait on this.

Origins

The specification allows for three options: no-preference, light & dark. However, one must not forget that not all browsers support the prefers-color-scheme media query, and that means there is a fourth option (or rather first option); where the browser does not (cannot) handle the media query.

In essence the CSS code can be split up into four sections:

/* [1] CSS of non-supported browsers here */

@media (prefers-color-scheme: no-preference) {
  /* [2] CSS for supported browsers where the user does not have a preference */
}
@media (prefers-color-scheme: light) {
  /* [3] CSS for supported browsers where the user wants a light theme */
}
@media (prefers-color-scheme: dark) {
  /* [4] CSS for supported browsers where the user wants a dark theme */
}

The existence of no-preference is odd¹ to me. Theoretically this one hands the preference over to the website author, allowing them to adopt their own preference. (Also, theoretically, a website could produce 4 different UI - but logically ... why? I also could not get any of the 6 browsers I use to trigger the no-preference query. Also - doesn't the website author have control anyway?)

The logical choice, thus, is binary: either light or dark. Naturally this would be brand based or some other definition, whatever, the point is that the website author will have a default position of their own, also binary light or dark.

user wants →<br>website has ↓(not supported)no-preferencelightdark
lightlightlightlightdark
darkdarkdarklightdark

If you simplify the logic table above you get a binary option, for the website author, that can be refactored into one of two methods:

  1. Default render a light page, with a dark override, but only if the user browser preferences a dark color-scheme
  2. Default render a dark page, with a light override, but only if the user browser preferences a light color-scheme
/* In this example the website author prefers a light theme, and places:
  [1] CSS of non-supported browsers here,
  [2] wich is also the CSS for supported browsers where the user does not have a preference,
  [3] wich is also the CSS for supported browsers where the user wants a light theme
*/
@media (prefers-color-scheme: dark) {
  /* [4] And then the CSS for supported browsers where the user wants a dark theme comes here */
}

Obviously, if the website author prefers a dark theme he would use an inverse approach:

/* [1] [2] [4] */
@media (prefers-color-scheme: light) {
  /* [3] */
}

Refactoring

Back to thinking about @Carl-Hugo's work been adapted to work with dark mode - if one applies the coding practice that one should never write the same piece of code twice, there is a whole bunch CSS that gets repeated (not that that code got written twice - but that principle applies both to writing and the consumption of memory when run) ... comparing the two CSS files to each other there is a bunch of duplication except in elements of color.

Generally speaking, if you look at CSS there are 3 core concepts:

Assuming we want to keep the Geometry (which is after all what the 10 year old concept of responsive design is all about) and the Type Faces unchanged, then all we really need is a deltas/differences package that can be used to offer a solution based on the concept of supplying the original CSS whole, and then toggle into dark with only the deltas.

Hypothesis

Given the logic table discussed prior, and the refactoring methodology also covered (that one only needs the deltas), and the fact that that the alternative color could be either light or dark, my hypothesis was that I could use all of the original Bootstrap code, but also needed to isolate the differences (specifically the color choices).

So, I set out into the code to strip all but the color elements - but run into three problems.

  1. How would I compile it? (Remember that I did not want to modify the core code.) The answer was to create a _variables-dark.scss, with only the color items in it;
  2. and, initially I just took all the variables and added a -dark suffix ... until I got to .table-dark ... ummm ... .table-dark-dark ... nope; so there was a naming issue;
  3. also, was -dark appropriate? What if, in third party theming, the primary color was dark, and that the prefers-color-scheme optioned (or deltas) was light? In essence the "alternative" color.

And so ... this proof of concept would attempt to prove that light default (because Bootstrap 4's default is light) and dark deltas was possible. And that a dark-then-light variant was simple to create with a selector, variable or mixin. (No attempt was made to offer a dark-then-light variant as the method would be evident in the PoC and that theme builders could just modify variables to achieve that.)

The question of compiling was easy enough to resolve; while I was extracting all the color variables, I needed to test the color combination - but all the Bootstrap code wasn't written for *-alt variables, so I had to map them back. Another file, _variables-map-back.scss, thus maps back the *-alt to non-alt. e.g. $body-bg-alt: #000 !default; and then $body-bg: $body-bg-alt;. Unintentionally I'd created a whole dark theme. I called it bootstrap-night.scss. (More on that later.)

The next phase was to look at how to use these new *-alt variables inside the core code... answer: I could not.

The *-alt SCSS includes

Contributor @ntkme weighed in on Issue #27514 and offered 4 ideas:

  1. Option 1: Wrap all Bootstrap code, twice over: one as normal, unfiltered, and then a second time with a @media (prefers-color-scheme: dark) {} query.
  2. Option 2: Drop .*-dark classes everywhere.
  3. Option 3: Support a build with different colors that's optimized for dark mode in two independent stylesheets
  4. Option 4: Fully switch to CSS variables

I pondered these - and (in my mind) responded¹:

  1. Option 1: Great idea! Except ... all of Bootstrap? Seems wasteful since the only things changing is the colors.
  2. Option 2: Oh No, oh no no no. And how would the HTML look like??? Oh my... JS to enumerate all classes and add ‘-dark' to the class names.
  3. Option 3: Brilliant idea! Use @Carl-Hugo's theme, and then instead of loading light as unfiltered and dark as a prefers-dark use @tomayac‘s two CSS with JS fallback. Works just fine (except the considerations already mentioned). Just needed a working example.
  4. Option 4: The work required to do this would be no more and no less than what I'd already done in creating the *-alt set ... except I was concerned with browser support (as already mentioned).

But @ntkme seeded an idea and this is where I excel at - taking other's seeds and growing them (thought-wise that is).

And so, I set out to build four variants of (or methods to achieve) "dark mode" support, namely:

  1. bootstrap-night.scss - this one (as already mentioned) was created accidentally in testing the color combination - but I also wanted to build a working prototype - this is after all a proof of concept. So, I'd prove this works. It also shows @tomayac's work beeing applied to Bootstrap.
  2. bootstrap-nightfall.scss - this one was seeded from @ntkme's Option 3 ... but instead of doubling up on the CSS, the alternative add-on would only contain the deltas/differences.
  3. bootstrap-nightshade.scss - this one was seeded from @ntkme's Option 2 ... but with one major difference. Instead of adding -dark to each CSS element just create one over-arching dark class on the <html> tag. Then the CSS would have html.dark xxx {} selectors, again only for the deltas. Plus, I needed to prove the CSS would work by writing some JS to toggle the dark class in and out as the user changes preference with a listener.
  4. bootstrap-dark.scss - the grail stylesheet - this one was seeded from @ntkme's Option 1; traditional Bootstrap with a @media (prefers-color-scheme: dark) {} query that would present only the deltas in one self-contained, easily ported, no additional JS, solution. The perfect solution, albeit without the flexibility of the Bootstrap-Night example.

(Mind you; I named these after the fact.) Anyway ... No. 1 was already built, but No's. 2, 3 & 4 had something in common: The deltas. I needed to build the deltas without modifying the core code. Painstakingly I copied each scss include and edited all the non-color elements out and then also pointed all the color elements to their *-alt variables. 3 days later it was done. Thankfully I did not encounter the snag I did when I attempted to dark-mode-alize Foundation - where I found several instances where color was coded directly into the includes and even the mixins. This made it that I would have to in essence duplicate the majority of Foundation 6 SCSS and then edit out hard coded colors and replace with SCSS variables - sure doable, but next version updates to the core would require a redo and I was just not up for that. The other irritation of F6 was the SCSS var like $white were littered everywhere - so in making dark variants for Foundswatch the var $white are contain black CSS colors and visa-a-versa. Sure, workable but confusing as hell.

I did however come across one mixin that needed to be re-done, _forms.scss, where the color, background-color, border-color and box-shadow where all coded in using non-alt SCSS variables, so needed to create a form-control-focus-alt mixin that then used the *-alt vars, and then find where form-control-focus was called and change that too. I also needed to re-do the _functions.scss file with color-yiq-alt, color-alt, theme-color-alt, gray-alt & theme-color-level-alt function using the *-alt vars. Not a biggie.

The end result was that I now had a set of SCSS includes that when compiled offered me a deltas package that I could use in all three remaining variants. That and the map-back functionality I mentioned meant 4 very different ways of producing the same color combinations.

The Variants

Bootstrap-Night

Method 1

bootstrap-night is a stand-alone CSS that is essentially just a themed version of Bootstrap. One can use it as stand-alone theme, similar to all the themes on Bootswatch, but since it is in essence exactly the same as default Bootstrap (in terms of styling sans color) it can be used as an alternative light/dark combination with the original default theme.

The essence of this code is simple: Use two fully themed copies of Bootstrap, one light and one dark. Each <link rel="stylesheet" would have different media type attributes, leveraging the prefers-color-scheme filter.

To do this use the following code:

<!-- Bootstrap CSS -->
<!-- Inform modern browsers that this page supports both dark and light color schemes,
  and the page author prefers light. -->
<meta name="color-scheme" content="light dark">
<script>
  // If `prefers-color-scheme` is not supported, but @media query is, fall back to light mode.
  // i.e. In this case, inject the `light` CSS before the others, with
  // no media filter so that it will be downloaded with highest priority.
  if (window.matchMedia("(prefers-color-scheme: dark)").media === "not all") {
    document.documentElement.style.display = "none";
    document.head.insertAdjacentHTML(
      "beforeend",
      "<link rel=\"stylesheet\" href=\"bootstrap.css\" onload=\"document.documentElement.style.display = ''\">"
    );
  }
</script>
<!-- Load the alternate CSS first ... -->
<link rel="stylesheet" href="bootstrap-night.css" media="(prefers-color-scheme: dark)">
<!-- ... and then the primary CSS last for a fallback on very old browsers that don't support media filtering -->
<link rel="stylesheet" href="bootstrap.css" media="(prefers-color-scheme: no-preference), (prefers-color-scheme: light)">

The first <meta name= ...> assists the browser in rendering the page background with the desired color scheme immediately. Thomas Steiner (@tomayac) discusses this in his article "Improved dark mode default styling with the color-scheme CSS property and the corresponding meta tag".

The <script> bit adds a bit of JavaScript that will inject the default (light) CSS into the html header with no media filter before the other two stylesheet declarations. This will force the browser to load the default CSS at the highest priority. Read Thomas Steiner's (@tomayac) ".. Hello darkness .." article on that.

Bootstrap-Nightfall

Method 2

The basic premise of bootstrap-nightfall is that it's built as a Bootstrap add-on. (Where the principle of "add-on" is an additional functionality requiring the original Bootstrap to be in use and then the add-on adds or modifies functionality and/or UI.)

In essence it can be used in a pure add-on mode to override Bootstrap color styling to give you a dark only theme.

<!-- Bootstrap CSS -->
<link rel="stylesheet" href="bootstrap.css">
<!-- Boostrap add-on -->
<link rel="stylesheet" href="bootstrap-nightfall.css">

But, knowing what we do about the media="(prefers-color-scheme: dark)" filter we can amend the header and include bootstrap-nightfall as a "dark scheme only" add-on.

<!-- Bootstrap CSS -->
<!-- Inform modern browsers that this page supports both dark and light color schemes,
  and the page author prefers light. -->
<meta name="color-scheme" content="light dark">
<!-- Load the primary CSS first ... -->
<link rel="stylesheet" href="bootstrap.css">
<!-- ... and then the alternate CSS first as a snap-on for dark color scheme preference -->
<link rel="stylesheet" href="bootstrap-nightfall.css" media="(prefers-color-scheme: dark)">

There is however one significant draw back that makes this option not feasible¹! The dreaded FOUC! Well - technically it is not a full "Flash Of Unstyled Content", and I'm only getting it in FireFox 76 (at the time I wrote this), but Safari and Chrome don't flash. By "flash" I mean that when the user is in dark-mode and the page loads, it shows the light scheme first, for a sub second, and then transitions to dark. This sub-second motion looks like a flash... each and every page... that I find super annoying.

There are techniques that can be used to remove the FOUC, but I personally don't like this approach and will not address this problem. It is after all a Proof of Concept, and the intent is to prove that this methodology works, not that it's a viable.

Bootstrap-Nightshade

Method 3

Initially when I set out on this PoC I did not intend to create bootstrap-nightshade. The principle behind being that dark mode could be driven by the addition of a class in the content that would define light / dark UI and some underlying code to allow a user to toggle that preference. But then I felt I needed to address the toggle button question and this methodology seemed the best to illustrate that (more on the toggle switch later).

The construction of the CSS file starts with the inclusion of all of the default Bootstrap, and then an inclusion of the deltas in a nested SCSS selector, like so:

@import x;  //** all of bootstrap **

html.dark {
  @import y;  //** all `*-alt` color stuff, a.k.a. deltas **
}

And so, with this one file one can just replace the Bootstrap CSS with the bootstrap-nightshade CSS.

<!-- Bootstrap CSS -->
<link rel="stylesheet" href="bootstrap-nightshade.css">

However, by itself, this will do nothing as the underlying Bootstrap original is active by default. To toggle over to the dark variant one needs to write some JavaScript to trigger this transition. (On a side note: There is a large body of recommendations to remove Bootstrap's dependency on jQuery - specifically those wishing to use it in React, AngularJS or Vue.js - but this PoC is based only on version 4.5.0 of Bootstrap, that is dependent on jQuery, so I've coded this toggle requiring jQuery.) The following code is used to toggle a dark class in the <html> tag:

<script>
  $(document).ready(function(){

    // function to toggle the css
    function toggle_color_scheme_css () {
      // get the current mode
      $mode = (window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches) ? 'dark' : 'light';
      // amend the html classes
      if ($mode == 'dark') {
        $("html").removeClass('light').addClass("dark");
      } else {
        $("html").removeClass("dark").addClass('light');
      }
    }

    // initial mode discovery & update button
    toggle_color_scheme_css();

    // update every time it changes
    if (window.matchMedia)
      window.matchMedia("(prefers-color-scheme: dark)").addListener( toggle_color_scheme_css );

  });
</script>

Why the <html> tag and not the <body> tag is covered by Geoff Graham in "HTML vs Body in CSS".

Of significant consideration here is that the dreaded FOUC is back - on all browsers! This is because the code is only executed once jQuery loads, thus the page already rendered, and the flash will most certainly occur when the user preferences a dark mode.

The toggle switch

As I was researching this, I came across a myriad of content describing the philosophy and how-to of getting a toggle switch going and it <u>seemed</u> to be the norm. But the more I read the more I realized that these recommendations pre-date the advent of the prefers-color-scheme media query. Even more recent content, like Chris Coyier's "Let's Say You Were Going to Write a Blog Post About Dark Mode" recommends that "Dark Mode could (should?) be a choice on the website as well" suggesting that even though one leverages the prefers-color-scheme because the user's browser specifically asked for it by specifically opting in to dark mode on his OS, or by browser theme (as Firefox has the option to) - that that user may want your site to be light. Umm... I call BS¹ on that. I just don't get¹ the philosophy of a user, at OS level, saying that they want everything they do on that platform to be dark - except on your website. I don't even think it's a thing¹, and unless I see a study showing how many users set dark mode preferences on OS level and then expect light on given websites - across many websites - I'm not buying into the toggle switch idea. IMHO I think it's a legacy thought conjured up before prefers-color-scheme media query was adopted by the W3C - and that that thinking is no longer needed¹. (You don't see cars being pulled by horses anymore, do you?) The only "experts" still peddling the toggle switch are yet to catch up with the feature set... I read one saying that he disagreed with Apple pushing its agenda on this (even though its standardized by the W3C, and adopted by Microsoft, Mozilla and Google), and another saying that his users demand it - then looked at his dark variant and wanted the light one too - only because that dark mode implementation could have been classified a sin¹. Developing dark mode websites needs specific thinking around supporting two color variants and there are several other considerations to account for other than CSS. (More on that later.)

If you look at sites that use the prefers-color-scheme media query correctly - like Twitter and StackOverflow - there is no toggle switch.

Nevertheless - I myself used a toggle switch in some of the test pages ... but that was because I wanted to toggle between the Night theme and the default Bootstrap theme to see if the colors worked. I never intended to develop a persistence layer for it. But in the case of NightShade I pondered persistence and how that would play out in a scenario where the user OS and the user interaction were not aligned and how to handle it as a logic experiment. So I prototyped it and it's code is viewable in the test-nightshade example.

Bootstrap-Dark

Method 4

bootstrap-dark, is in my opinion the grail CSS¹, the one I would use. It can be used as a drop-in replacement for the original CSS. No additional code, no additional add-ons, and works on all the supported browsers.

Internally the CSS is composed of all of the original Bootstrap and then a single (prefers-color-scheme: dark) media query. Within that query would be all the color CSS elements, just in the alternate colors (deltas).

To use it, simply replace the Bootstrap CSS stylesheet:

<!-- Inform the browser that this page supports both dark and light color schemes,
  and the page author prefers light. -->
<meta name="color-scheme" content="light dark">
<!-- Bootstrap CSS -->
<link rel="stylesheet" href="bootstrap-dark.css">

Utility classes

There are also a small set of utility classes embedded in the bootstrap-dark CSS.

Display

Use the d-light-* and d-dark-* classes to isolate parts of your site to only display on certain color-scheme modes, like this:

<!-- this will only display in light mode, and only on browsers that support it -->
<span class="d-none d-light-inline">Try this website on dark mode.</span>
<!-- this will only display in dark mode -->
<span class="d-none d-dark-inline">Thank you for saving your eyes in dark mode.</span>

You can also use these to display when not on certain color-scheme modes, like this:

<!-- this will only display in browsers that do not support `prefers-color-scheme` -->
<span class="d-no-preference-none d-dark-none d-light-none">Your browser is old!</span>
Selection

The ::selection CSS pseudo-element applies styles to the part of a document that has been highlighted by the user (such as clicking and dragging the mouse across text). This variant also contains an additional import file _dark.scss that adds styling for this pseudo-element.

SCSS mixins and variables

I mentioned all the *-alt variables earlier, but there are two more entities worth a mention:

  1. The $color-scheme-alt variable in the _variables-alt.scss class sets the alternate color mode. This can be one of light or dark. If you're a theme builder and your theme is primarily dark, then set this to "light" and populate all the *-alt variables with your light color selections.
  2. The prefers-color-scheme mixin. This mixin creates the media filter based on your selection. It's only parameter is the color mode, which can be one of: "no-preference", "light" or "dark". If you're building custom elements or additional CSS you can use this in SCSS like this:
.my-mighty-widget {
  color: red;
}
@include prefers-color-scheme(dark) {
  .my-mighty-widget {
    color: blue;
  }
}

Method 4b

bootstrap-unlit produces exactly the same output as bootstrap-dark in supported browsers, but falls back to the darker theme on browsers that do not.

To use it, simply replace the Bootstrap CSS stylesheet:

<!-- Inform the browser that this page supports both dark and light color schemes,
  and the page author prefers dark. -->
<meta name="color-scheme" content="dark light">
<!-- Bootstrap CSS -->
<link rel="stylesheet" href="bootstrap-unlit.css">

Internally the CSS is composed by the same techniques as Method 4, but also leveraging the map-back techniques in Method 1 and described in the Hypothesis, or rather a more elaborate 3 step approach of saving the original color, mapping back the -alt's to the original, and then reseting the -alt's to the saved originals - thus swapping them around. Not elegant, I know, but this is just a POC after-all.

[Note this method is superfluous as it produces the same result as Bootstrap-Dark, but I felt remiss if I didn't prove that one could fallback to the dark theme.]

Where's the proof?

On the Github Pages page: https://vinorodrigues.github.io/bootstrap-dark/

And the winner is ...

Sadly, there is no winner here (for Bootstrap 4). Unless these concepts are brought into the core of Bootstrap there will always be the challenges of maintaining the code against current release (and I want to make it clear that I have no intention to update this body of work to keep up with Bootstrap). The problem with Information overload means that this body of work will probably not be read by those who would benefit from it.

Whether or not dark mode will be included in Bootstrap 4 or even 5 is unknown as I am not active in the core group - Issue 27514 is marked [<span class="badge badge-warning">V6</span>] so I assume the intent is not for [<span class="badge badge-info">V5</span>]. (Reading the existing work committed to the v5-dev branch it seems focused on removing the dependency of jQuery ... but it's early days).

Nevertheless, a PoC will need an outcome:

The No's

The Yes'

Can you use this?

HELL YEAH! Go ahead - I made this for learning; mostly me, but also for others. I would have released it as public domain if not for some of my references requiring share alike clauses. So MIT it is.

Github

If you're a theme builder or want to use its principles in your own project you'll need to have Git and Node installed.

  1. Fork or download the repository: git clone https://github.com/vinorodrigues/bootstrap-dark.git
  2. Install Node dependencies: npm install
  3. Modify _variables.scss and _variables-alt.scss in the scss sub-folder.
  4. Run npm run build to build your theme. (This uses NPM Scripts to build the css files. You may need to install some of the dependencies as "global".)
  5. The compiled code will be in the dist folder.

CDN

You can also hotlink the theme via CDN with jsdelivr.com.

You can access the theme CSS file from the GitHub release:

NPM

Developers can include the scss and dist folders into their own projects with:

npm install bootstrap-dark-4

Feedback

If you have useful feedback drop me an "Issue" on the GitHub Issues page.

<p align="center" style="text-align:center;display:block;font-size:75%">&diamondsuit;</p>

But that's not enough

[The proof of concept pertaining to Bootstrap ends above - but I felt remiss if I did not cover the fact that Bootstrap can only do so much - that there are other considerations to get Bootstrap (or any other UI/UX) to work in dark mode.]

Having a dark mode enabled stylesheet is not magically going to make your website "dark mode enabled". The fact is this that true dark mode does not stop at the CSS - there are other considerations.

If you pause for a moment and think about the underlying enabler for this - the CSS media query - its inception was as far back as 1994, was drafted into the W3C in 2001 and only ratified into formal specification in 2012. (See the "Media queries" Wikipedia page.) It took the browser vendors and the W3C 18 years to make this a standard. Nevertheless, this gave birth to the wonderful philosophies of "Responsive Web Design" ("RWD") and "Mobile First". According to Wikipedia - the first demonstration of this technology was publicized by Cameron Adams on 21 September 2004 in his blog entry "Resolution dependent layout". If you look back at my dissection of what CSS does for you¹ - Geometry, Type Face, & Color - RWD is thinking about and manipulating the web-page Geometry. Another fascinating aspect is that mobile browsing surpassed desktop browsing back in May 2016 (according to StatCounter) - but strangely it's hovered in the 50-55% range since then (see "Mobile Vs. Desktop Usage ..").

The point is - even though RWD has been around since 2004, and mobile took over the market share of browsing in 2016 - today there are still websites that do not account for mobile users, and that adoption of these technologies has been "slow and steady" but not quite there yet. In my opinion this is because:

Even those who do adopt these philosophies find it does not work for them all the time - mostly because they forgot, or don't know, about that additional thinking required to make RWD work. The other challenge is that RWD (and it's after-thoughts) is relatively technical and requires both efficient coding and great design to implement correctly - either as teams or in the rare case, and if you can find them, a web developer that is both good at code and design. It's not uncommon for a Marketing department to grab an SVG export straight out AI and publish it on a website (for example, a logo file 785,189b in size), whereas a coder applying compression techniques would get the same file 0.24% of the original size! (Yes, that's a "." (decimal) in front of the "24", or 1,863b in size.) In the case of a single file this is neither here or there as mobile bandwidth can process 1Mb rather quickly - but if this is that same non-thinking¹ being applied to, say, 100 images on one page, makes for a very sluggish and slow mobile web experience.

Dark mode adds another layer to the coding / UX / design thinking: how should the website look in alternative color combinations?

This can be split into 2 considerations:

  1. What's inside the web content,
  2. and, what's outside the content.

If you consider what else has color in content the answer is simple: The media. For inside content, that's easy: Images and Videos, but outside the domain of your own content color also plays a role - for example in your favicon or application icon.

Handling these two is specific to your content and varies from site to site - but what I can offer is some recommendations and considerations for you to ponder on.

Images

For most sites images make up the bulk of the content, and so these need special attention - getting images to work in dark mode is complex and requires considerable attention - and image formats can add, or alleviate, complexity. I can offer a rule of thumb that may simplify the selection:

Generally, images that work in light mode may not work in dark. Photos taken in bright conditions may appear overly bright in a dark environment.

Mark Otto suggests in "CSS dark mode" to darken the image by adding an opacity, thus allowing some of the dark background to darken the image, SCSS like so:

@media (prefers-color-scheme: dark) {
  img:not([src*=".svg"]) {
    opacity: .75;
    transition: opacity .5s ease-in-out;
    &:hover {
      opacity: 1;
    }
  }
}

Grgur Grisogono suggests in "How to Set Up Dark Mode for Images" to apply a greyscale filter, SCSS like so:

@media (prefers-color-scheme: dark) {
  img:not([src*=".svg"]) {
    filter: grayscale(50%);
    transition: all .5s ease-in-out;
    &:hover {
      filter: none;
    }
  }
}

Melanie Richards suggests to adjust the brightness and contrast with a filter, SCSS like so:

@media (prefers-color-scheme: dark) {
  img:not([src*=".svg"]) {
    filter: brightness(.8) contrast(1.2);
    transition: all .5s ease-in-out;
    &:hover {
      filter: none;
    }
  }
}

All these techniques assume the image to show in its original state when hovered on.

The other way to approach images is to use the same techniques one employs to deliver responsive images (as in different images for different content widths), but apply the media query to the prefers-color-scheme filter, HTML like so:

<picture>
  <source srcset=" xyz-dark.jpg" media="(prefers-color-scheme: dark)">
  <img src=" xyz-light.jpg" width="320" height="240">
</picture>

This last method provides you far more flexibility - in that you can have completely different images, remembering to keep the dimensions the same. The downside is that you need to create (and maintain) twice as many images.

SVG

SVG's offer unprecedented advantages for web developers - and the overall topic of SVG is not in the scope of this paper. But what I do want to show you is how to leverage SVG to define images with different outcomes based on the user prefers-color-scheme media query.

[Remember that the below techniques not only apply to color - but can also be used to manipulate color-like concepts like opacity, and even things like visibility.]

In-line SVG

HTML 5 introduced in-line SVG - basically just drop in the compliant SVG code into the HTML instead of a <img> tag and it would render the image. Best of all it becomes part of the DOM so can be manipulated with JavaScript and, and be styled with CSS.

<!-- CSS style for svg -->
<style>
  @media (prefers-color-scheme: dark) {
    #my-image #xyz {
      fill: #0000ff;
    }
  }
</style>
<!-- Inline SVG -->
<svg width="32" height="32" id="my-image"
  viewBox="0 0 100 100" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <g id="xyz" fill="#101010" fill-rule="nonzero">
    <path d="M50,1.78e-15 C77.61,1.78e-15 100,22.39 100,50 C100,77.61 77.61,100 50,100 C22.39,100 0,77.61 0,50 C0,22.39 22.39,1.78e-15 50,1.78e-15 Z M50,6 C26,6 6,26 6,50 C6,74 26,94 50,94 L50,75 C64,75 75,64 75,50 C75,36 64,25 50,25 L50,6 Z"></path>
    <path d="M50,25 C50,25 50,75 50,75 C36,75 25,64 25,50 C25,36 36,25 50,25 Z"></path>
  </g>
</svg>

Embedded SVG

The traditional approach to embedding images can also be used - with SVG that can be the usual <img> tag, or <object>, or <embed>. As with HTML, SVG 2 supports the "class" and "style" attributes on all elements to support element-specific styling. It also supports general styling with a <style> tag.

Given that the browser renders the SVG we can assume that a browser that supports the prefers-color-scheme media query can also handle it in the SVG itself.

<?xml version="1.0" encoding="UTF-8"?>
<svg viewBox="0 0 100 100" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <style>
    @media (prefers-color-scheme: dark) {
      g path {
        fill: #efefef;
      }
    }
  </style>
  <g fill="#101010" fill-rule="nonzero">
    <path d="M50,1.78e-15 C77.61,1.78e-15 100,22.39 100,50 C100,77.61 77.61,100 50,100 C22.39,100 0,77.61 0,50 C0,22.39 22.39,1.78e-15 50,1.78e-15 Z M50,6 C26,6 6,26 6,50 C6,74 26,94 50,94 L50,75 C64,75 75,64 75,50 C75,36 64,25 50,25 L50,6 Z"></path>
    <path d="M50,25 C50,25 50,75 50,75 C36,75 25,64 25,50 C25,36 36,25 50,25 Z"></path>
  </g>
</svg>

And embed in one of four ways:

<embed height="32" width="32" type="image/svg+xml" src="image.svg" />
<object height="32" width="32" type="image/svg+xml" data="image.svg"></object>
<iframe height="32" width="32" frameborder="0" src="image.svg"></iframe>
<img height="32" width="32" src="image.svg">  <!-- `<img>`+`prefers-color-scheme` only works on Firefox and Opera -->

[Please note that <img> this does not work on all browsers yet, so is not a recommended approach.]

Favicon

With the concepts of dark mode being so young - and the prefers-color-scheme media query, technically still in draft - browser support has some way to go in supporting everything dark mode can be. One of these is the favicon concept - this image is used mainly for two things; a) bookmark icon, and b) browser tab icon. With devices in dark mode, that bookmark can be on a dark background, or browsers in dark mode can have a dark tab -- in which case a dark colored favicon can become difficult or impossible to see.

At of the time of writing this paper no browser supports prefers-color-scheme media query for the favicon. This is slowly changing, and according to Thomas Steiner this will soon be supported in Chrome.

Until then you'll need to design your favicon in a way that works for both light and dark browsers. Some of the techniques include using colors that work in both modes or creating images with a frame or contrasting color outline / contour.

Postface

The technology behind dark mode is young and has yet some way to go. As web developers, creating content that follows the rule of "always respect a user's color preference" takes a mindset change. Just like responsive web design requires a change in the way we think, write and test website geometry, dark mode requires the same considerations for website color.

The key to changing mindset is knowledge - the more we understand the concepts the more it becomes natural for us to think in terms of those concepts. As such I will close off this paper with a list of other resources that can help.

<p align="center" style="text-align:center;display:block;font-size:75%">&copy; 2020</p>