FadeOutThemeAnimation class

public sealed class FadeOutThemeAnimation : Timeline

<FadeOutThemeAnimation ... />

<StackPanel>
    <StackPanel.Resources>

        <!-- Theme animations like this can use the default behavior like
             this example or you could use properties like BeginTime and 
             Duration to tweak when and how quickly the animation occures. 
             If you want more control over a fade animation (e.g. just partial
             fade), you will need to use DoubleAnimation to animate the Opacity 
             property of the element you want to fade in or out. -->
        <Storyboard x:Name="EnterStoryboard">
            <FadeOutThemeAnimation Storyboard.TargetName="myRectangle" />
        </Storyboard>
        <Storyboard x:Name="ExitStoryboard">
            <FadeInThemeAnimation Storyboard.TargetName="myRectangle" />
        </Storyboard>
    </StackPanel.Resources>
    <Rectangle x:Name="myRectangle" 
               PointerEntered="Rectangle_PointerEntered" 
               PointerExited="Rectangle_PointerExited"   
               Fill="Blue" Width="200" Height="300" />
</StackPanel>

// When the pointer enters the rectangle space, the animation begins.
private void Rectangle_PointerEntered(object sender, PointerRoutedEventArgs e)
{
    EnterStoryboard.Begin();
}

private void Rectangle_PointerExited(object sender, PointerRoutedEventArgs e)
{
    ExitStoryboard.Begin();
}