public class CustomToggleSwitchButton : RadToggleSwitchButton
{
    private FrameworkElement thumb;

    static CustomToggleSwitchButton()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomToggleSwitchButton), new FrameworkPropertyMetadata(typeof(CustomToggleSwitchButton)));
        IsCheckedProperty.OverrideMetadata(typeof(CustomToggleSwitchButton), new FrameworkPropertyMetadata(OnIsCheckedPropertyChanged));
    }

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        this.thumb = this.GetTemplateChild("PART_Thumb") as FrameworkElement;
    }

    private static void OnIsCheckedPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var switchButton = (CustomToggleSwitchButton)d;

        if (switchButton.thumb == null)
        {
            return;
        }

        bool? isChecked = e.NewValue as bool?;
        if (isChecked == true)
        {
            switchButton.thumb.HorizontalAlignment = HorizontalAlignment.Right;                
        }
        else if (isChecked == false)
        {
            switchButton.thumb.HorizontalAlignment = HorizontalAlignment.Left;
        }
        else if (switchButton.IsThreeState)
        {
            switchButton.thumb.HorizontalAlignment = HorizontalAlignment.Center;
        }
    }
}