Gtk 3.18.* HeaderBar - background window bug fix

public class Window : Gtk.Window {
    construct {
        var color = get_style_context().get_background_color (Gtk.StateFlags.NORMAL);
        override_background_color (Gtk.StateFlags.NORMAL, { 0, 0, 0, 0 });

        Gtk.Box box = new Gtk.Box (Gtk.Orientation.VERTICAL, 0);
        box.override_background_color (Gtk.StateFlags.NORMAL, color);

        // code here
    }
}