Built in UI customization could potentially open up a whole new industry. I imagine a site like MSDN or TechNet for graphic and UI designers (something like http://www.wincustomize.com/). Microsoft could make design tool for this like Visual Studio is for MSDN.
The other benefit will surely be the speed of an integrated theme manager ( a more powerful one than there currently is). WindowsBlinds and StyleXP are slowing the system down.