while I possibly can not tell how the actual GTK Theme for Gnome3 will look like, I suppose that It will use the GTK-CSS Engine which just made a first release.
One might ask why the next Gnome should use an engine which just looks as crappy as that, but that becomes pretty obvious when you think about what CSS is.
CSS is the language used to layout XML document such as XHTML websites and thus it is flexible enough to support every possible design one might think of. Furthermore it is pretty widespread – probably not in the desktop world, but everyone only barely related with web design should have a pretty good knowledge of it.
And now lets look at CSS Zen Garden a Website created to showcase the possibilities of CSS – look at the themes on the right and how they can change the face of the website. Remember it is always the same physical structure! Now think all of these would be possible with gtk and just using one engine – pretty cool, right?