[callahad]

I'd expect controls for this to improve in the future (e.g., https://bugzil.la/1547818), but for now you can use this add-on to force a dark scheme: https://addons.mozilla.org/firefox/addon/dark-mode-website-s...

Or, in about:config, set the hidden ui.systemUsesDarkTheme to 0 for light, 1 for dark, and 2 for no preference