By default, the pixels of your screen are black. Lighting them with bright colours requires more power. Therefore, web sites with white backgrounds, that are used by millions of users (like Google), cause a considerable use of energy. The people behind the page Blacke had that in mind when launching Blackle. The page consists of the Google search engine, but with a mostly black interface. According to Mark Ontkush, if Google would have a black background, energy use of their users would go down by 750MWh. This claim is however valid for old, energy hogging CRT screens - if all screens were to be of the modern LCD type, the numbers would be different. And even if 750MWh would be a correct figure, it is negigible when count per user.
But initiatives like Blackle are interesting, because they highlights the energy use of tech companies. Although the internet society reduces energy use on a macro level through distance work, reduced use of physical materia, etcetera, internet tech companies have come into focus regarding energy use.
With soaring costs for energy use (web giants like Yahoo and Google report yearly 20% to 40% increases in energy costs, not counting the cost for their users), it is going to be interesting to see how companies like Google respond to the impact of their business. As a starter, Google are installing asolar power system at the Googleplex.