A couple of points:
I think you may be on the wrong email list / forum for this sort of question. You are asking about command lines and such, but this forum is about the D binding to GTK.
I just tried:
$ GTK_THEME=Adwaita gnome-calculator $ GTK_THEME=Adwaita:dark gnome-calculatoron Debian SID running GNOME and it worked exactly as expected.