Thats right, darker websites like this one use less energy than the common white background website, because dark pixels require less energy from your monitor than white pixels. Just by punching in a few numbers you can reduce the energy requirements of your website.
I’ll admit I had to estimate that 89% number, but I didn’t make all this stuff up. There is a great site called www.BLACKLE.com, who remind us that by taking small steps in our daily lives, we can save energy. That information informed the design of this very site and is why you see a predominantly dark background instead of that standard white (#ffffff) thats become default on so many design sites.
I find small little details like this to be incredibly interesting and important to me as a designer. I’m especially interested in issues of sustainability that go beyond putting little arrow icons on “earth tone” paper and strive to find innovative solutions for real world problems.