😆, Actually You have to do that by changing your Browser's Theme Mode{Automatic/Dark/Light} under Settings, It has nothing to do with the Theme.
How To > 👀 LINK -- https://pasteboard.co/7WckyFzg2GOn.jpg
..
BTW You can also try my other **theme** 🌈 >LINK
https://addons.mozilla.org/en-US/firefox/addon/gradient-x-love