Provide an unified interface for setting GTK theme
Currently GTK themes are set with the xsettingsd config. Because of that setting different themes for GTK2 and GTK3 does not work properly. GTK2 by itself is old and at some point needs to be deprecated completely (not yet). Therefore disallowing setting separate themes for GTK2 and GTK3 is not a big deal (as mentioned above it does not work anyway).