For now, the Firefox-CSS is based on the existing repository Godiesc/Chameleons-Beauty and the Firefox theme Beautiful GX Proton.

The goal is to make a CSS without third party stuff, but for now this approach must suffice.

Set toolkit.legacyUserProfileCustomizations.stylesheets = true under about:config in Firefox.

Set the theme in Firefox to the default dark theme that comes with the installation.

Goto to the Root Directory of the Default Profile of Firefox. Can be found via about:profiles in Firefox.

cd ~/.mozilla/firefox/profiles/<default-profile-folder>/

Clone the repository into the profile directory as follows:

git clone https://github.com/Godiesc/Chameleons-Beauty chrome_orig
ln -s chrome_orig/chrome chrome
git clone https://github.com/tonigineer/firefox-css chrome_edit

Now edit the userChrome.css of the Chameleons-Beauty repository:

echo "@import url('../chrome_edit/changes.css');" >> chrome/userChrome.css

Restart Firefox.