Use LESS instead of CSS for styling

This pull request adds the [`less`](http://lesscss.org/) library and converts the existing default `css` to `less`. Currently, the `celestrium-defaults.less` is compiled client side (via the `less.js` library that is included). For deployment, we can change this to compile `celestrium-defaults.less` to `celestrium-defaults.css` locally if we decide that performance is a huge deal.