depends
if you want to stick to the OS out of the box, you can check out the pocket pc themes section and look under manila themes
if you want to flash a rom you should have your device unlocked
http://forum.ppcgeeks.com/showthread.php?t=43266
afterward click on cdma upgrade forums and have your pick of roms...
but it depends on device
are you using a vzw touch pro or a sprint/alltel/telus/uscellular touch pro