Having recently moved to a tablet device running the kindle app, the white background – being actively emitted was just too jarring. So I switched to white text on a black background.. which at first felt strange, now seems pretty comfortable, but actually now appeals to me at a more fundamental level.
When viewing a normal book it is generally black text on white paper. The words which represent that content which we are actually interested in are only discerned through their negative space. Our eyes receive all of the reflect light from the entire page, with the exception of the content.
I seem to rather like that conceptually, and it may help stop our house from over-flowing with dead-tree based reading matter. (shame you don’t seem to be able to lend a book to a friend or donate it to a charity shop once you’ve finished it)