Now that CSS and Javascript are so widely used, it would be interesting for OS vendors to provide a default OS theme along with their packaged browser, that allowed some kind of sane default for web applications to use. You could use a native skin, or load your own CSS to customise it. Microsoft kind of tried this with ActiveX controls, and then Java Applets tried it, and then Flash was fun by ultimately not a good idea, and now we're here with thousands of themes to choose from and a rough set of guidelines that some follow and many don't.