http://freestyler.ws/style/12191/google-reader-larger-serif-wider-plus-color
Seeing http://freestyler.ws/style/11803/google-reader-contrast-improvement-for-articles by Kent Fredric@userstyles inspired me to update my own Reader style, http://freestyler.ws/style/2464/google-reader-larger-and-serif-plus-wider, with a little color. My style uses green for unread and blue for read entries, preserves the blue border for selected entries (to aid with keyboard navigation), and yellow for hover highlighting. As with my older style, this one uses a larger serif font (Georgia) and widens the body for each entry to almost 100%.

NOTE [22Jun2010]: I've noticed lately that Google Chrome 5 has an issue involving Google Reader and some styles such that the first time--and sometimes even the second/third time, too--you go into Reader during a browser session, the color highlighting (as well as all of the other "stylings") is lost; subsequent visits have no issue (yet). Chrome 4 doesn't have this problem, and neither does Firefox (any version [...yet]). No known reason for it; doesn't seem to be an auth problem. I'll post here if I come across an explanation/resolution.

Updates...
26Aug2009 [v2]: remove border for entry title when expanded; remove yellow background for selected entry in list view; lighter blue background for read entries in card view; minor tweaks, plus hide the 'like' stuff (which, if you actually like, then you can remove the code at the end)