A developer is literally a hop, skip and a jump away from figuring out how to make OS7 themes by using Theme Builder 6. I've been informed that the developer has finally gotten Theme Buider 6 to recognize the Bold 9900 and may have even successfully loaded a 9700 theme to a 9900 and gotten the phone to acknowledge it. Of course that's assuming a few tweaks occurred on the 9700 theme, but that I'm actually uncertain about at this time.
For now though, the writers at BBOS will keep our eyes peeled for a leaked, or even official, release of an OS7 theme. But that doesn't mean you can't send in a few hints and tips to us in the mean time!