How to configure and test a new theme before going live

Kindof a dumb/noob question...

How can I test, configure, and customize BP Daily for use on my site before actually switching themes?

I run a community of about 300 members and blogs on MU 2.9.2 and BP 1.2.3

Obviously, I can't just activate BP daily for a few hours in the middle of the night, mess with it, then reactivate my current theme before morning.

Most of the theme-previewing-type plugins i've seen aren't advanced enough.

How does everyone else do it?

Thanks!