I know this will not help you with making your own......but....For complete themes that will skin everything including the calculator, Win Mobile, and the phone take a look at here:
http://www.spbsoftwarehouse.com/unithemes/?en
You will have to join the SPB club...which is free...to download them for free....or pay $10 for each on. I have been using them for a while now. I currently have the silver one installed. Here is a sampling of what some of them look like:
Artic Blue
Silver
Tech
