public IReadOnlyList<Element> GetSelectedElements()

{
  //Get element's selection count.
  //Count the number of selected elements on the active layout view
  LayoutView activeLayoutView = LayoutView.Active;
  if (activeLayoutView != null)
  {
    var selectedElements = activeLayoutView.GetSelectedElements();
    ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show($@"Selected elements: {selectedElements.Count}");
  }
}

{
  //Set the CIM halo properties of a north arrow.

  //Reference the first selected north arrow element
  var northArrow = LayoutView.Active.GetSelectedElements().OfType<NorthArrow>().First();

  //Note: Must be on QueuedTask.Run
  //Get definition of north arrow...
  var cim = northArrow.GetDefinition() as CIMMarkerNorthArrow;

  //this halo symbol is 50% transparent, no outline (i.e. 0 width)
  //First construct a polygon symbol to use in the Halo
  //Polygon symbol will need a fill and a stroke
  var polyFill = SymbolFactory.Instance.ConstructSolidFill(ColorFactory.Instance.CreateRGBColor(0, 0, 0, 50));
  var polyStroke = SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.BlackRGB, 0);
  var haloPoly = SymbolFactory.Instance.ConstructPolygonSymbol(polyFill, polyStroke);

  //Set the north arrow definition of HaloSymbol and HaloSize 
  ((CIMPointSymbol)cim.PointSymbol.Symbol).HaloSymbol = haloPoly;
  ((CIMPointSymbol)cim.PointSymbol.Symbol).HaloSize = 3;//size of the halo

  //Apply the CIM changes back to the element
  northArrow.SetDefinition(cim);
}