Awesome

Adwaita for Steam

A skin to make Steam look more like a native GNOME app

Steam updates may reset theme: While the installer will patch steam files in order to theme the interface, these files may be reset by steam updates. If this happens, you will need to reinstall to repatch them.
Rounded corners: Not all elements can be rounded, use Rounded Window Corners Reborn extension on GNOME.
Steam website pages (Store/Community/Profiles/etc): No longer themable with our current method.
Position of notifications: Doesn't seem to be possible to change.
Height of game list entries: Doesn't seem to be possible to increase.

Python 3
The skin is created and tested mostly on the Linux version of Steam. Steam's new UI has largely unified platform differences, so other platforms should work, but are not a priority.

<details><summary>Color theme previews</summary>

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git clone https://github.com/tkashkin/Adwaita-for-Steam
cd Adwaita-for-Steam
./install.py

Argument	Short	Required Values	Description
--help	-h		Show help message
--list-options	-l		List available themes and extras
--color-theme	-c	Colortheme	Change color theme
--windowcontrols-theme		auto / adwaita / windows / macos	Change window control buttons style
--windowcontrols-layout		auto / gnome / pantheon / windows / macos / [custom]	Change window control buttons position and order
--custom-css			Enable custom css
--extras	-e	Extra	Enable one or multiple theme extras
--target	-t	normal / flatpak / [custom dir]	Choose target location for install (default: both)
--uninstall	-u		Uninstall theme

# List options
./install.py -l
# Install with customizations
./install.py -c nord -e login/hide_qr -e library/hide_whats_new

<details><summary>Custom CSS</summary>

If you wish to include your own custom CSS, see custom and use the --custom-css flag.

./install.py --custom-css

</details>