I've been experiencing a strange problem with my themes. Sometimes 7 will stop displaying the desktop background. It just becomes a black screen. I can try changing the desktop image, refreshing, show/hide icons...nothing works. The background just stays black. If I change themes, the UI color will change, but the background will stay black. The only way I can fix the issue is to reboot.
Note, this almost always happens during the transition from Aero to Basic (e.g. if I run a program that doesn't support Aero), but will also happen randomly.