Porting CSS class name docs from the old site and placing it in a new th...

...eming category. Fixes #59: Document CSS

I made up `type="basic"` so there might be a better of handling that. I made this an entry rather than a page because I thought theming deserved its own category and listing in the sidebar. Otherwise I think this content will be hidden.

The content itself is mostly the same as the old site but I altered it a bit.