My small modification of default Firefox UI (for versions with enabled native vertical tabs) "System theme" not supported , only normal (Light/Dark/etc.) Tested on official Firefox 139 and optimized for default UI scale.

1. Enable toolkit.legacyUserProfileCustomizations.stylesheets in about:config page.
2. Move userChrome.css to ProfileFolder/chrome/ (See path in about:profiles page).
3. Restart browser and use!