tl;dr if you are unfamiliar, but you can basically css style parts of the browser that aren't modifiable with extensions or themes!
it's locked behind an about:config option though (toolkit.legacyUserProfileCustomizations.stylesheets)
i think it's like that because they don't really guarantee that the things you css aren't going to change between updates, though i feel like styling body in certain urls is pretty safe
anyway, the entire code for this is here if you're curious what that looks like in practice! (or if you want to modify it to do your own thing~)
