The designers are very much involved in GTK+ development. Their designs guide what features (widgets, themability, performance) we include, improve and drop in GTK. Their opinions on themability also influence what we think about it.
Of course, they don't have the final say and when we disagree, we try to find a solution that works for all of us.
In the context of themes, the GNOME project considers themes a bad thing, because you cannot advocate a unified design if you don't know how things are going to look in the end. This is not an uncommon view for desktop projects - neither OS X, iOS, Windows nor Android come with multiple designs.
For GTK itself, the users outside of GNOME (Windows, OS X, Unity, Sugar, XFCE) have so far made a strong point towards keeping themability a part of the toolkit.
But I'm saying it all the time, and am repeating it here: the GTK toolkit is defined by the people that take an interest and participate in developing it. And those people are currently almost 100% coming from the GNOME side. And as long as that is the case, expect GTK to be more and more the GNOME toolkit and less and less something else.