Mark Otto takes us on a tour of the CSS of GitHub.com. So great to read a straight-talking bit like this, warts and all.
This is 90% the same as we do things at CodePen. One major difference is that they combine all their CSS into two files and just serve them on all pages. I suppose that way once you've visited any page of GitHub.com, you've browser-cached all the CSS you'll ever need there. I have tended to go with a global.css for everywhere and a section.css, which leverages browser cache for the bulk of it while keeping the total CSS weight per-page down.