This style is a great idea, only problem is that the hover doesn't work for the "contacts" page. If you add this: tr.y:hover{background-color: rgb(255,235,134) !important;} it will highlight contacts you hover as well. You might want to add this to the style...
Edit: having it green is probably more logical, so you'd have this instead: tr.y:hover{background-color: rgb(205,243,159) !important;} By the way any reason for using rgb() instead of a hex value?
Comments in the code would be useful, so we know what id/class relates to which parts. makes modifying a *lot* easier :) Besides that, it's really useful - because of the number of feeds I have on IG, a little part of it isn't visible on my screen. Your CSS helps countering that - thanks!
About Gmail: Inbox with new style on Dec 29 2006
About Clean Google IG on Oct 30 2006