Odd, that should work in GTK3 as well, and does IME...
I've definitely had issues with it, I can't remember if it was just certain (weird) apps like Firefox, or maybe Xfce or KDE specific issue, or possibly just one of GTK+2 or 3 apps not updating when a dual-support theme is selected. Anyway, whenever it happens/happened, restarting the app always fixed it.