Allow switchable themes

[Raynes]

I do not want to have a pastie-like color theme selection menu. I think that's over the top and pointless. What we need are two good themes, one light, one dark. I think I'm on the right track by writing a tomorrow-night-bright color theme for pygments. It works well with the site. I think we should also implement a tomorrow-night-light theme as well and allow for users to switch between them. I think that it should be a query argument and not a toggle because a) not many people are going to use it b) we need to avoid cluttering the pages with buttons.