Can you do anything with the phone when its on? Like go into Settings etc? If so then perhaps you can remove the theme in Manage Applications. If you can't do anything with the phone when its on then sounds like a job for three button recovery and a factory reset.

Your contacts should be in the cloud, so no worries there right?

I'm guessing you have no Titanium Backup if you're worried about your apps? Might be a painful learning exercise about backing up here...

The theme maker hangs out at XDA-Developers, so perhaps asking there will get you sorted.