add background class to GladeDesignLayout children

gladeui/glade-design-layout.c

@@ -973,10 +973,13 @@ glade_design_layout_add (GtkContainer *container, GtkWidget *widget)
 {
   GladeDesignLayout *layout = GLADE_DESIGN_LAYOUT (container);
   GladeDesignLayoutPrivate *priv = layout->priv;
+  GtkStyleContext *context = gtk_widget_get_style_context (widget);
 
   priv->child_rect.width = 0;
   priv->child_rect.height = 0;
 
+  gtk_style_context_add_class (context, "background");
+
   gtk_widget_set_parent_window (widget, priv->offscreen_window);
 
   GTK_CONTAINER_CLASS (glade_design_layout_parent_class)->add (container,