Implement theme switcher

[jelbourn]

We're going to shamelessly steal the picker from https://fonts.google.com/

This is lower priority than the main docs content

[jelbourn]

Oh, also we should make it so that the different themes are separate files that we lazily load when the theme changes (instead of loading them all at once).

[Splaktar]

Theme switching seems to be implemented now:
#136
#188

This can probably be closed?