I have tried almost every theme I can find, with the same issue. When I upload the theme to /my-templates/ the thumbnail in the backend appears as a broken link. When the theme is applied, it appears that no stylesheet is applied. I’m at a total loss for understanding what is going on here.
Please check it out at blinktechnologies.com/bbpress
It has been fixed in the latest development version of bbPress, but the packaged version 1.0.2 still creates the folders with the wrong permissions. Sorry about that.