In January 2007, a theory was was proposed in ecoIron that a black version of Google will save 750 Megawatt-hours of energy every year. This was based on the the article by Energy star that a white pixel needs more energy than a black pixel. According to the article, a white pixel needs 74watts of energy wheras a black pixel needs only 59 watts. With 200 million queries made each day, Mike of ecoIron proposes that the difference will be very large if google decide to change its background to black. Initially there was skepticism that this will affect the readability of the pages, which is not entirely correct. For printed media, which is a reflective media, a light background will be more readable. Hence you need highest contrast to decrease the strain on your eyes. But for computer screens, too much light can give headaches and visual trails. Hence a dark background is much more suitable for reading, as far as computer screens are concerned.
If you are into web design and development, then this article by Vorlath will tell you the importance of using a dark coloured background over a light background.