This will let the theme unify the appearance of this button with
the window buttons in 'traditional' title bars.
gtk_widget_set_valign (button, GTK_ALIGN_CENTER);
context = gtk_widget_get_style_context (button);
gtk_style_context_add_class (context, "image-button");
+ gtk_style_context_add_class (context, "titlebutton");
icon = g_themed_icon_new ("window-close-symbolic");
image = gtk_image_new_from_gicon (icon, GTK_ICON_SIZE_MENU);
g_object_unref (icon);