GUI_APP_MAIN {
  // Define the colors you want to your window by calling the
  // many ::SColorXXX_Write(Color) functions before running
  // your app
  ::SColorFace_Write(Color(r, g, b));
  ::SColorText_Write(Color(r, g, b));
  ...
  MyApp().Run();
}

  class MyApp : public TopWindow
  {
   public:
     MyApp() {}

     // Paint override
     void Paint(Draw& w) {
        w.DrawRect(GetSize(), Color(r, g, b)); // <= enter your background color here
     }
  };

GUI_APP_MAIN
{
  MyApp().Run();
}