Hey Tres,
in my opinion, environment variables are about the worst possible option for this sort of thing.
If you are linking against GTK, the easiest way is to just ask GTK itself if you need to know
the theme name:
g_object_get (gtk_settings_get_defautt (), "gtk-theme-name", &theme, NULL);
But I am not sure that the theme name is really needed. What information are you looking for
exactly ? And what decision are you going to make based on it ?