We use it frequently. It uses white screen which consumes higher power. Read the following.. If Google had a black screen, taking in account the huge number of pages viewed, according to calculations, 750 mega watts/hour of power per year would be saved"
In response, Google created a black version of its search engine, called Blackle, with the exact same functions as the white version, but obviously with lower energy consumption:
I dont think it has any effect, atleast in recent times. Recently I wrote that Google made its page black for one day to spread awareness of energy conservations. They did placed a note saying that making the screen black in no way saves energy (if I think the latest lcd screens atleast, if not the old crt) but it is just to spread the idea of energy conservation.
So making a site black saves power - I am not sure whether it is true.