Slowly but surely
As I have mentioned before, I'm working on a native theme for the Mac OS X port of GTK+ from time to time. I recently spent some time on the theme engine to make Devhelp look a bit better on Mac, with focus on some widgets that are very visible in Devhelp: notebooks, scrolled windows (and other widgets) with shadows, and scrollbars.
It's clearly very difficult to get things to look exactly right and in some cases I think you will have to use platform specific code to arrange widgets differently to get a really good look. BUT, nevertheless it's getting better:

