I just did a view source to see if I could determine that and found some interesting results.
It does appear to be custom CSS, and very minimal, but what surprised me was that the HTML was as minimal as possible as well. There are no HTML, HEAD, or BODY tags.
Maybe not technically valid, but an interesting way to save a few bytes.
It is valid HTML5. <html>, <head> and <body> will be inserted by any HTML5-compliant browser while constructing the DOM. However, it'll result in a broken page if parsed by an old browser.
As mentioned by orftz it is valid HTML5, but what I did find interesting is that the footer is being inserted with javascript, does anyone know the reason for that?
My best guess is that it becomes a client-side include, making it easier to change one file and propagate out to all the pages without any server-side processing. Seems a bit silly for so small a thing, though.
