I've been using the Preview(dense) theme and I don't have that problem at all. While the issue could be due to your eyes, it is most likely being exacerbated by viewing the site on a monitor where the contrast setting is set too low and/or the gamma is set too high. Granted, good universal web design should account for the fact that many people's monitors won't be ideally calibrated, but if you like the theme other than for that issue you might want to try adjusting your contrast and gamma settings.