Add new themes

Themes are always nice! This is a placeholder issue for theme proposals.

I'll probably need to write a guide on how to do it, but you basically just need to create a new file in styles/themes following the other ones that already exist, then register it in here.

New themes should be able to provide a good contrast (at least in respect to the containing page's background). This tool might help: https://wave.webaim.org