Not very convenient but using Stylish it comes down to adding entries into a text file:
https://github.com/stylish-userstyles/stylish/wiki/Applying-...
Re:UX Myths (2014)
by unexistance on Monday October 31, 2016 @02:56AM
(#12834742)
yeah, some sites will 'break', but not a problem to me so far...
I suspect it can be done via Firefox's userchrome.css file, but I never dabble in it
It's userContent.css ; userChrome.css is used to style the browser itself (the "chrome" of the browser, that is to say what is around the content: location bar, menus, etc. Yes the collision with this other Google browser is confusing these days, but Firefox/IE usage of the term predates Google Chrome, it's actually Google Chrome that hogged the term, not the other way).
But be careful, Electrolysis (multi-process firefox) broke userContent.css, if you are running it you'll have to either disable Electrolysis, or use Stylish. Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1046166
I like userContent.css, it's a single css file, easy to understand/sync. Hope a fix will arrive someday.
Is there a way to opt in to the site's color choices per domain?