gtk-key-theme-name on MacOS can't be changed via settings.ini
172f894 hard-codes the gtksetting gtk-key-theme-name to "Mac" when GDK_IS_QUARTZ_SCREEN by passing it as a parameter to g_object_set_data_full() right after the call to g_object_new() for the gtksettings object; that was consolidated with menu settings into the g_object_new() call in 88f16b.
The problem is that the settings.ini files are parsed during gtk_settings_init() and then any passed in properties are applied afterwards so if a user tries to set a different gtk-key-theme-name in a settings.ini it's effectively ignored.