private Polyline MakeArrow()
    {
        Polyline pl = new Polyline();
        pl.Style = Resources["ClosedTheme_Arrow"] as Style;

        return pl;
    }

private Polyline MakeArrow()
    {
        Polyline pl = new Polyline();
        //pl.Style = Resources["ClosedTheme_Arrow"] as Style;

        Windows.UI.Xaml.Media.PointCollection pc = new Windows.UI.Xaml.Media.PointCollection();
        pc.Add(new Windows.Foundation.Point(0, 0));
        pc.Add(new Windows.Foundation.Point(4, 5));
        pc.Add(new Windows.Foundation.Point(0, 10));
        pl.Points = pc;
        pl.Stroke = new Windows.UI.Xaml.Media.SolidColorBrush(Windows.UI.Colors.White);

        return pl;
    }

<Style x:Key="ClosedTheme_Arrow" TargetType="Polyline">
    <Setter Property="Points" Value="0,0 4,5 0,10"/>
    <Setter Property="Stroke" Value="#fff"/>
</Style>

<Setter Property="Points" Value="0,0 5,4 10,0"/>