Surely it makes no difference unless you are using a plasma screen or an old fashioned CRT. Anyone using a LCD (in other words nearly everyone) will be using the same amount of energy powering the illuminating backlight whether the screen is black or white. The energy needed to switch the pixels on and off is almsot zero in comparison. If you really want to save the planet, as New Scientist showed recently, just don’t use Google. Google uses huge amounts of energy to power the banks of high power computers and even more energy to keep them cool enough to operate. For more energy than if everyone used black pixels instead of white ones.