The theme PGF is using is Tarski.
Tarski is a WordPress theme created by Ben Eastaugh and Chris Johnson. As they say on their website, it definitly works right out of the box.
What we liked at first glance was the tasteful header artwork that was included , Tarski has numerous options accessible via a simple, easy-to-use options page. For the more technically-minded, Tarski provides a flexible basis for any number of alterations, from simple things like changing the header, to more complex structural changes. We are now on the latest release, Tarski 1.1.
The coolest thing we discovered is that the theme is named after a real person, Alfred Tarski, one of the greatest logicians of the 20th century, and the father of formal theories of semantics. He is best known for his groundbreaking theories of truth and logical consequence.