I've been planning to implement an alternate theme but haven't done it yet. Black on white is particularly irritating for people with astigmatism which appears to be a pretty big part of the population.
People with astigmatism (aproximately 50% of the population) find it harder to read white text on black than black text on white. Part of this has to do with light levels: with a bright display (white background) the iris closes a bit more, decreasing the effect of the “deformed” lens; with a dark display (black background) the iris opens to receive more light and the deformation of the lens creates a much fuzzier focus at the eye.
But nobody uses it.