Enable dark theme for users that prefer it

[bennypowers]

This PR causes the dark CSS to load for users who prefer dark mode. Closes #586

[arp242]

It's not very good though. Like I said in the issue, I really need someone to go over it for a bit.

[bennypowers]

IMO they could be addressed in a future PR

I could take a look at implementing an existing color scheme like catppuccin

[arp242]

IMO they could be addressed in a future PR

But in the meanwhile it'll look like crap for everyone with the system theme set to "dark". Especially since browsers don't have a particularly good way of enabling/disabling this per-site AFAIK it'll be quite jarring I think. That's my concern.

Maybe what we can do instead is add a dark/light switcher in the settings or on the top bar, and not watch for prefers-color-scheme: dark yet (until it's improved). This way people who really want it can easily enable it, but it's not going to be a jarring "wtf is this?!" for everyone else.

[arp242]

There's now an option to switch in the user settings (6a25298); the default is still just "light"; once a proper dark theme is available we can change the default to prefers-color-scheme.