The plethora of themes is one of the more appealing features of Windows 7 for customisation addicts. Indeed, there's so many options it can be tricky to know where to start.
The Engineering Windows 7 blog runs down the features added to Windows 7 that make it easier to customise all kinds of aspects of Windows, and share them with others. While you might not be inclined to create your own Windows 7 soundscape, the walkthrough on how to customise your themes using RSS is definitely interesting.
Creating, Saving, Sharing Themes in Windows 7 [Engineering Windows 7]