Quote Originally Posted by Stig
When I "learned" to make Powerpoint presentations early this year one of the rules we got was:
Always use a black background with white texts, it's easier for the eyes and people will stay focused.
I tend to use a yellow or broken white background, with a bad enough projector it tends to come out as a not-too-intense white, also easier on the eyes. I never tried the white text on a black background before (for presentations) it seems counterintuitive for some reason.

The main downside of a black background is that colours don't come out as much, which can be fairly annoying when programming.