Some sites are unusable what that setting. For example, it makes the voting arrows on HN invisible. And it makes the chess boards on lichess.org invisible.
Is there a way to opt in to the site's color choices per domain?
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?