the directory for current theme used by Gtk

[Krister <thekswenson gmail com>]

I'm trying to get pyinstaller to import the minimum number of files
necessary for a Gtk theme:
https://github.com/pyinstaller/pyinstaller/issues/3084.

I can get the current theme name using the code:

        import gi
        gi.require_version('Gtk', '3.0')
        from gi.repository import Gtk
        print(Gtk.Settings.get_default().get_property("gtk_theme_name"))

I then can search the system directories for the theme name (on Ubuntu
17.10 it's /usr/shar/themes), append the theme name, and just import
everything under that directory.

Is there a more direct way to get the directory path of the theme
being used by Gtk?