In fact, there have been numerous studies that show we're much better at reading white-on-black text, but because of historical reasons (paper) many of us are conditioned to prefer black-on-white. That 63% would be higher with a different cultural background.
Note that not all such studies look at the same thing. I should've been more precise: on paper and with training, people were much faster at quickly reading a page if it was light-on-dark than dark-on-light or dark-on-white.
