Background on one monitor only.

I have a pretty simple 2 monitor setup. With C8 the (default) grey background is only visible on the main monitor and the background on the second monitor retains it’s themed colour. The main application window can be dragged across to the second monitor but the “desktop” behind it stays desktop coloured. In earlier versions I could get the whole 2-monitor background to be the same colour. Is this a fault, a “feature” of Aero themes, or am I missing preference setting?