It is a remake of the current MediaWiki Phase 3 layout skin. See the Layout design document.
(Note that you can't see it directly because HTML files are sent by the server as text for security. You'll need to download it.)
Comments are welcome on the talk page.
Note this is still a work in progress!
Experimental live version
Log in to the test wiki and select Paddington. It sort of works, but it's very incomplete and buggy.