I have been using the themes that Rori and upwardspiral have created and put on the market place. The themes are applied using the theme chooser that comes with the rom. Just look them up on the market place and see if anything catches your eye.
I know its another GB theme but you should check this one out and see what you think. I just installed it last night and everything is running flawlessly. Its a Steel Blue theme and it is simply amazing looking. The icons in the notification bar are a little different from what I have seen in most other themes with the exception of the 3G icon which I can deal with.