I used ThemeGenCE 2.5 which you can get from this page:
http://www.codeppc.com/telechargemen...themegence.htm
(V)eGa also has a quick tutorial on this here
http://forum.ppcgeeks.com/showthread.php?t=22087 but I had to pick QVGA P+L instead of the normal QVGA he says to use so that I could set all the images correctly.
This thread should probably go to the Theme forum though.