Just wanted to say that you don't even need to be running the ROM for which you're creating the theme. I downloaded CM7 a couple days ago, here's what I did:
1. Downloaded the CM7 ROM
2. Created my desired theme before even flashing CM7
3. Created a stock CM7 flashable theme just in case I wanted to revert later
4. Copied CM7 and both themes to the sd card
5. Flashed CM7
6. Flashed my theme
7. Rebooted into CM7 and my theme was in place
I recently created a theme for a friend who was using a phone I have never used. Probably a little risky but I was comfortable doing it.