Yes, that’s right — no more boring Gmail! If your account has them (apparently they haven’t fully rolled out yet) you’ll find the new themes under Settings > Themes.
The best part so far is that the Gmail UI itself is a little cleaner no matter which theme you choose, very nice!
If you don’t see them in your account yet the Official Announcement has a larger sample pic.
I’m using the Mountains theme right now, but there is more than one I like. Which do you prefer?