Some time ago Peter made the comment that in the final release he'd look into including Theme Chooser, but apparently that never panned out. He's back at school and sounds like he's going to have a busy semester and has put some of his Android work on a much lower priority. This is is fine by me, as his ROM sort of comes across as a no-thrills type of ROM anyway. However, apparently his last release is open source so from my understanding someone could implement Theme Chooser at their own peril. I have no idea how to do any of that, but I would expect to see some custom ROMs based off of GPA.
Someone did post a theme in his GPA17 thread at XDA. It's not really to my liking - personally running the Honeybread theme on the Kush Deck 1.3d ROM. I've used Peter's ROM and really liked it but I go through phases and am currently stuck on Kush Deck.