PickerHandler.Mapper.AppendToMapping("Background", (handler, view) =>
{
var border = new Android.Graphics.Drawables.GradientDrawable();
border.SetShape(Android.Graphics.Drawables.ShapeType.Rectangle);
border.SetStroke(4, Android.Graphics.Color.Red);
border.SetCornerRadius(12);
handler.PlatformView.Background = border;
});
Is this the result are you looking for?