well i prefer dark background since it match perfectly for almost all colors. of course, most websites you see have dark backgrounds are it blends everything well compare to light background which sometime hard to match up.
Actually, the color of the background depends entirely on the content (its type) and the mood you want to create. Also with a white background for e.g, creating transparent gif's becomes a lot easier I think.
of course, most websites you see have dark backgrounds are it blends everything well compare to light background which sometime hard to match up. 
