How do I get gtk3 themes to load? For example when I use Xfce, the Gnome 3 theme defaults to something ugly, where as gtk2 themes can be changed using Xfce Settings Manager → Appearance → Style. But firing up gnome-control-center and navigating to Appearance → Adwaita doesn’t change the appearance of evince when in Xfce. Here is a screenshot which demonstrates the difference:

http://picpaste.com/gtk3_vs_gtk2-tdrlvKCs.png

The same happens in fluxbox. To change different gtk2 themes for gtk2 based apps in fluxbox, I now use mate-appearance-properties. What app do I use to change gtk3 themes for gtk3 based apps like evince or nautilus when I am using a non-gnome3 based window manager?

As far as I know, there is no tool to only switch the gtk3 theme. gtk2 theme only means gtk3 reverts to the distro default. One thing you can do is find a gtk3 theme you like, that matches the gtk2, or whatever your preference. Copy over the gtk3 directory from that theme into the theme directory of your preferred theme.