Maybe not 1.21 gigawatts, the energy needed to power Doc Brown's fabled Flux Capacitor -- but nearly 750 megawatts, according to the folks at EcoIron. The back-of-napkin math behind the idea is this: white pixels take more wattage to display than black pixels. Something like 200 million queries come Google's way each day. Google is white. That means Google is being displayed about 550,000 hours per day, cumulatively.
Assuming most people display the page full-screen, on both CRT and LCD monitors, changing the Google homepage background to black would save about 8.3 Megawatt-hours per day, or 3,000 per year. At 10 cents per Kilowatt-hour, that works out to approximately $75,000/year in energy savings -- all from changing just a few lines of HTML code.