Theme support

[irskep]

I would like to try making my own CSS theme from scratch, but I don't want to tread on your preferences. I also think people like to personalize things, and themes are a nice way of doing that.

The simplest way I can think of to do themes is to add a config var THEME, the value of which is inserted into the CSS file path. So I would move all the existing CSS from static/css/*.css to static/css/main/*.css, and then make my own directory static/css/xxx. The line count of the project would be the same.

Is this something you'd consider merging?