BuddyPress.org

If I’m not wrong, you can just make the Buddypress Default theme available to users in the Site Admin–>Themes section of the dashboard, and disallow other themes.

I’d probably create a new child theme (blank, basically, so it’s identical to the Default Buddypress child theme) and name it after my site, then make it the only one available.

You can change the default WPMU theme if you want more than one available via a code snippet somewhere here in the forums that I have since lost the link to (check the stickies?) or with this paid plugin (which is in and of itself not worth the signup fee I don’t think).

This free plugin may offer the same functionality but I haven’t tried it.

Alternate: Don’t know if this still works. I haven’t tried it.