there was a discussion about this in the Dump a while back
basically, a "dark" webpage doesnt save any energy for the end user, with the possible
exception of people browsing with newer monitors that have high dynamic contrast ratios (my monitor has a static backlight, so it uses just as much energy regardless of whats on the screen)
but any energy savings from that would be outweighed by the dozens, maybe hundreds, of extra servers used to host all the alternate "dark" searches, for example BPLED claims to have saved 254 kWh of electricity, if we assume the server and necessary equipment use 500w of power themselves (probably a very conservative assumption) then as long as the site has been up more than 20 days BPLED has used more energy than it saved
the one truth of the site is this, buried near the bottom of their "About Us" page
In addition its better for your eyes. Certainly when you are in a dark environment.