code: private void definePanels(UIDefaults d) {
          String p = "Panel";
          d.put(p + ".contentMargins", new InsetsUIResource(0, 0, 0, 0));
          d.put(p + ".background", new ColorUIResource((Color) d.get("control")));
          d.put(p + ".opaque", Boolean.TRUE);
      }
docs: Initialize the panel settings.
  
  @param d the UI defaults map.