GTK_THEME= doesn't work for gnome-terminal?

If I just want to see what a gtk3 app looks like with a theme other than my default, I can run:
GTK_THEME=Adwaita mousepad
or
GTK_THEME=Ambiance mousepad
or
GTK_THEME=Greybird mousepad

Other gtk3 apps behave the same way but if I try the same approach with gnome-terminal, it still opens with my default gtk3 theme, the one I point to in my ~/.config/gtk-3.0/settings.ini. Does this happen to anyone else?

16.04 Lubuntu Openbox session, fully updated. (Please note that GTK_THEME=theme_name app_name won't work with 14.04. It needs a newer gtk3 version than what is default in 14.04.)