public IReadOnlyList<Element> GetSelectedElements()

//Get the selected layout elements.

var selectedElements = layoutView.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 element (assumption is it is a north arrow)
Element northArrow = LayoutView.Active.GetSelectedElements().First();

//Perform on the worker thread
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 defintion 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);
});