[Feature Request] Custom CSS field

[TheDMV2]

Instead of just having the two inbuilt themes, if you add a field in the admin UI section that will allow you to add CSS, we could start really customising our libraries!

Basically, the same as how Jellyfin does, is what I'm suggesting.

The extra style tag is added right at the end, just before the </body> tag.

You can see I'm pulling in a remote theme for my Jellyfin instance (though I'll copy it and make it local soon), and we'd be able to do the same.

Super simple :)