title: Reveal highlight in RevealBackgroundBrush doesn't work
body: <p>I try use RevealBackgroundBrush, but Reveal Highlight doesnt work. </p>
  
  <p>RevealBorderBrush hightlight is work.</p>
  
  <p>RevealBackgroundBrush in sdk style work too (for example "ButtonRevealStyle"), but if i copy ButtonRevealStyle in app resources and apply style for button RevealBackgroundBrush doesnt work.</p>
  
  <p>I reinstalled windows, and try use brush in another uwp projects, but is doesnt work</p>
  
  <p><strong>I try use CSharp code:</strong></p>
  
  <pre><code>private void RevealGrid_Loaded(object sender, RoutedEventArgs e)
  {
    var grid = sender as Grid;
    grid.Background = new RevealBackgroundBrush() {
      Color = Color.FromArgb(80, 0, 0, 0),
      TargetTheme = ApplicationTheme.Light,
      AlwaysUseFallback = false
    }; // grid.background
  }
  </code></pre>
  
  <p><strong>And i try use Xaml:</strong></p>
  
  <pre><code>&lt;Grid Height="48" VerticalAlignment="Top" Loaded="RevealGrid_Loaded" &gt;
     &lt;Grid.Background&gt;
        &lt;RevealBackgroundBrush AlwaysUseFallback="False" TargetTheme="Light" Color="#33818181" /&gt;
     &lt;/Grid.Background&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p><strong>Details:</strong></p>
  
  <ul>
  <li><p>Visual Studio 2019</p></li>
  <li><p>SDK 10.0.18362.0</p></li>
  <li><p>Windows 10 Pro 1903 (18362.239 build)</p></li>
  </ul>