It depends on the type of screen. If I remember correctly the iPad's screen turns on pixels to block the back-light. That means a black screen uses slightly (very slightly) more power than a white screen. Some screen technologies do the opposite, turn off pixels to let light through. They would use slightly more power to show a white screen.
Either way, it is the back light that consumes most of the power when your iPad's display is turned on. Turning down the brightness, even a smidgen, will have a much larger impact on battery life than choosing a dark or light background. You are better off just keeping whatever background pleases you, and remembering to turn down the brightness when you don't need it.
It's also easier on your eyes when the iPad's screen is not dramatically dimmer or brighter than he surroundings, and you'll own your eyes a lot longer than you will the iPad.
