Add theme toggle for dark/light mode #364

Pretty self-explanatory. The main reason is when you don't want to have the same settings as your system preference, or, in my case, when the browser overwrites that setting (librewolf).

Librewolf does this for privacy reasons (fingerprinting), but it can't be disabled per-site. I am currently using dark reader to somewhat have a dark mode experience, but every time the page reloads (basically on every button click), I get a light mode FOUC.