it is much easier to change the look of windows than a Blackberry. you dont really have to do much of anything. you dont need any software that isnt included in the operating system. You can also customize the OS while it is running. This is not true of blackberries. Everything requires an additional piece of software, and time and testing.
What does your windows theme have that isnt native to the OS. How is it changed from normal? And also why are we comparing the blackberry to a PC anyway...? For a truer comparison, why not try PocketPCMall.com - Welcome!
look a site with both free, and not free themes and software for Windows Mobile. (a much closer approximation to the blackberry than a PC)