In the "old" days when I worked at Digital, we used to use VT100 character cell terminals. I always chose white text on a black background (this was a setting) because I knew that the black parts of the screen meant that the electron gun of the CRT display was turned off, and as a result the only current expended was being used on the white characters. This was partly environmental but mostly because I wanted to avoid seeing the 60 Hz refresh of a totally white background. I'm very sensitive to screen flicker and would set the refresh rate at the highest setting possible. Perhaps this is a symptom of my Asperger's syndrome but that is a topic for another post.
My sister informed me of an alternative site to Google that had a black background. The name of the site is Blackle, and if you use a CRT there is a certain amount of truth to the premise that a black background will save energy. Of course most clever people have already embraced the more energy efficient LCD screens which rely on polarizing filters and a backlit screen which is "on" all the time. So I will give my sister a shout-out for discovering this but in reality it is mostly hype. Of course a quick search on Google produced a real life demonstration, Here is the video: