BuddyPress.org

It’s actually quite easy to do this. I recommend getting comfortable with the functions that BP introduces to the templates, which can usually be found in buddypress/bp-core/bp-core-templatetags.php. There are some others, but those will be your friends.

Some of them are wrappers for WPMU functions, and others are BP specific. Each plugin also has a *templatetags.php file that includes more functions to component specific output.

Spend some time looking through the bphome and bpmember themes to get an idea of how those functions are used, and you’ll be on your way to making a custom theme in no time.