public CIMSymbolReference BackgroundSymbol {get; set;}

//Apply a background color to the map frame element using the CIM.

//Perform on the worker thread
QueuedTask.Run(() =>
{
  //Get the layout
  var myLayout = Project.Current.GetItems<LayoutProjectItem>()?.First().GetLayout();
  if (myLayout == null) return;

  //Get the map frame in the layout
  MapFrame mapFrame = myLayout.FindElement("New Map Frame") as MapFrame;
  if (mapFrame == null)
  {
    //TODO Handle null mapframe
    return;
  }

  //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);
});