There is a significant physiological advantage to black text on white background. A bright background causes your pupil to constrict, which reduces the active area of your lens focusing light onto your fovea. Using less lens area reduces the affect of any lens imperfections, which results in a sharper image. If your lens is nearly perfect, you likely don't notice a difference. If you have significant imperfections, the difference is probably very noticeable.
Edit: The effect is similar to a pinhole occluder used by an ophthalmologist to test your eyes without the affect of refractive errors from imperfections in your eye lenses. See https://en.wikipedia.org/wiki/Pinhole_occluder
Edit: The effect is similar to a pinhole occluder used by an ophthalmologist to test your eyes without the affect of refractive errors from imperfections in your eye lenses. See https://en.wikipedia.org/wiki/Pinhole_occluder