I'm not a graphic designer by trade, but elements of graphic design come up often when you produce a web site. I'm always conflicted about whether to use a dark background with light text or vice versa.
With paper, it's obvious that dark text on a light background is easiest to read.
When I'm working all day in emacs, though, black on white text is painful. I either have to darken the background to a lighter gray, or sometimes I reverse the colors to a light gray on black or dark gray.
I've always been able to work much longer without strain with a light on dark background, but I'll be the first to admit it doesn't look as warm and pleasing as dark text on light.