code: protected Color getHeaderBackground() {
          Color c = UIManager.getColor("SimpleInternalFrame.activeTitleBackground");
  
          return c != null ? c : UIManager.getColor("InternalFrame.activeTitleBackground");
      }
docs: Determines and answers the header's background color. Tries to lookup a
  special color from the L&amp;F. In case it is absent, it uses the
  standard internal frame background.
  
  @return the color of the header's background