You may find problems with themes as applications migrate from gtk2 to gtk3.
Once an application migrates to being gtk3 based, it will (often) no longer support gtk2 themes. The default fallback gtk3 theme tends to be (old old old) Raleigh.
You will need to keep an eye out for two things, your favorite themes being ported to gtk3 and applications that port to gtk3. I'm pretty sure mousepad has already made the transition.