Stylesheet themes

This is how I propose we handle multiple stylesheets per theme.

Basically, I moved everything that wasn't application.css.scss or the themes folder into the base folder.

Next I added another link tag to the layout, but based on the current theme.

Have a ./base directory and a ./theme directory would let us use `require_tree ./base` and `require_tree ./theme/my_current_theme`, instead of `require_tree .`, since the later solution would include *all* themes, not just the current one.