LighterTheme skins Trac, slightly, for a "lighter" look-and-feel. It provides a narrower (940px) main content div; fewer dark lines in the top (mainnav and metanav) banner; bigger buttons (using CSS from Twitter Bootstrap); rounded corners on input fields; and the "Ubuntu" font used on Launchpad.
Screenshots are available on the plugin's Trac Hacks page: http://trac-hacks.org/wiki/LighterTheme