Alexander Larsson has blogged about some of the GTK+ work taking place for Microsoft Windows (Win32) support.
"Then I forward ported all the work to the current Gtk 3 tree. It was mostly trivial, but one thing that changed a lot in Gtk3 is theming. The old windows theme was mostly a custom themeing engine, but in Gtk+ 3 we want to use engines less in favour of CSS. So, I had to completely redo the windows theme using CSS...There are clearly still some issues that need fixing, but it works impressively well for just being some CSS."