public CIMSymbolReference BackgroundSymbol {get; set;}

{
  //Apply a background color to the map frame element using the CIM.
  //Note: Must be on QueuedTask.Run
  //Get the map frame's definition in order to modify the background.
  var mapFrameDefn = mapFrame.GetDefinition() as CIMMapFrame;

  //Construct the polygon symbol to use to create a background
  var polySymbol = SymbolFactory.Instance.ConstructPolygonSymbol(
                  ColorFactory.Instance.BlueRGB, SimpleFillStyle.Solid);

  //Set the background
  mapFrameDefn.GraphicFrame.BackgroundSymbol =
                                    polySymbol.MakeSymbolReference();

  //Set the map frame definition
  mapFrame.SetDefinition(mapFrameDefn);
}