BuddyPress.org

The nice thing about themes, they are just html. Which is free to look at and examine. Chances are if you take a look at the source, and spend a few minutes in an image editor you will find this “theme” pretty easy to replicate.

What will be much harder to do is the quality job Andy has done integrating bbpress inside of buddypress. This blend on buddpress.org is seamless, and very difficult to achieve at this point without a lot of “deep intergration”.

But take a look at the source, and I think you will see it holds most the answers.