Awesome
Nylas Arc Dark Theme
Arc Dark theme for mail client Nylas N1
Install
Download ZIP-archive and extract. Run Nylas N1, select Edit - Install Theme... and choose folder.
For KDE
Better use Arc Dark KDE pack and run this command, after install:
sed -i 's|2F343F|383C4A|g' ~/.nylas/packages/arc-dark/styles/ui-variables.less