How do you change themes in PivotX?

I know this is probably a really stupid question, but I can't figure it out for the life of me. I'd really like to port my WordPress theme to PivoxX, but I don't know how to make PivotX look at my theme (or the other two that are included by default, for that matter). I've done some hacking on the default theme (though, my theme really needs to be rewritten from scratch; the default theme doesn't scale different screen sizes well at all), but I'm sure it would be really disappointing if I forgot to back it up and wrote over it during the next upgrade. I have beta 11 (installed locally).