The new Gmail Preview(dense) Theme is just too light for my eyes. On my 27" screens I have to squint to see which mails are read vs unread.
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.
Something is also off with the typography; the line height (leading) makes it very difficult to scan your messages for some reason.
The contrast between the grey and white in such a small space makes it so the eyes can immediate tell the difference between the two.
Same here. I've been using Planets theme for a while specifically because it's immediately obvious what's read and what's unread.