Based on the excellent OS X Style for New Google Reader by scudda@userstyles with a few small modifications.

Made the bar below the entries the same height as the bar below the tree on the left
Added hover and selected backgrounds for the tree
Eliminated the small amount of space between the top bar that shows the title of the feed and the bar that collapses the tree
Made the title of a feed in the top bar white when looking at the feed directly (not in a folder)
Made any feeds not grouped into a folder show the orange RSS logo rather than the folder logo

There might be a few others. I'll probably be making more slight modifications to this over time. Only tested in FF3 so might not be as good in FF2.

See also OSX Spinner for New Google Reader for a nice spinner for the new Google Reader.