When I added the Google Reader Widget to this site, I chose the style "none", which inherits the site's style and layout. However, it did some odd things with margins, so I wanted to style it manually. Google doesn't provide any styling documentation, but a quick inspection using Opera's "Show Structural Elements" option in the Author Mode button revealed class names. Here they are:
/* google reader widget */
div.reader-publisher-module {} /* container for list */
div.reader-publisher-module ul li {} /* list items */
div.f {} /* "Read More" link */
Also note that links in the Google Reader Widget don't have escaped ampersands, which produces invalid XHTML 1.0.