I hate skinnable applications, too, as the vast majority of skins look awful and work worse. However, the Orbit theme (available at mozdev.org, which is down for me right now) is both at least reasonably attractive and works well. A bigger plus would be if it didn't have the author's signature at the top. In order to fix that, put this in your userChrome.css file:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

menubar > .toolbar-orbit-holder > .toolbar-orbit-stack > .toolbar-orbit-box > .toolbar-orbit-right {

  background-image: none !important;

}