[CyberMonkey]

There are many factors you must look at.
When you say theme are you talking about a simple Windows Mobile theme or changing the interface?

Changing the interface you will need something like SPB Mobile Shell, PointUI, iFonz, or even Wisebar.

Searching google for any of those and you will find them easily.

Any more questions, just search the forums here (there are some threads that already discuss interfaces and themes) or ask if you can't find it in any of the threads here.