Here is one for the jailbreak/Home brew crowd. One of the largest sweeping accusations that users give to not adopt Windows Phone 7 is that the OS is too closed and that modification of themes seem impossible.
Strides have been made by Dave Amenta and other developers to change the accent color (tile colors) for devices, however it appears that (at least) HTC devices have full customizability of the home screen color – including background, accents, and tiles. Ideally, users can now have access to the registry and can use Advanced Task Manager. This may not be HTC specific, but let us know in the link provided. Have fun!
Full instructions at XDA-Developers here.