Preprocess CSS files

[marzer]

As of c90fa96 the CSS files have been 'unrolled' such poxy's overrides can be injected earlier, rather than making tweaks after-the-fact with clunky selectors and !important shenanigans. The unfortunate caveat of this is that it means there's now no longer one-CSS-file-to-rule-them-all, meaning more HTTP requests. I'd like to address this before the next release.

Relates to the discussion in #5.