Black background in Gnome control center

I must have messed up something, because all of the subwindows in Gnome control center have black backgrounds. It makes them quite hard to read. I use Ubuntu 13.04 with Gnome Shell 3.8. Changing themes does not help. What CSS property controls the background of these windows? What has gone wrong here? BTW the main Gnome control center window is fine (i.e. has a white background). Thanks, Jos.