Use alternative style sheets for night/retro mode

[strugee]

https://developer.mozilla.org/en-US/docs/Web/CSS/Alternative_style_sheets

[strugee]

So the main issue here is that you can't select more than one alternate stylesheet, and that's a usecase I want to support. Honestly if it was easy I'd do this feature but I'd have to rework a bunch of stuff to account for the fact that with alternative style sheets the browser would only consider one "active" at once... it's probably not worth it.