title: UWP NavigationView header not changing with theme
body: <p>When changing the RequestedTheme, the header background of the NavigationView does not update.
  Tried to use a headertemplate, but it didn't solve anything.
  Same problem with the window titlebar.</p>
  <p>before switch:
  <a href="https://i.stack.imgur.com/k7Zya.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/k7Zya.png" alt="before switch" /></a></p>
  <p>after switch:
  <a href="https://i.stack.imgur.com/iAFyw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iAFyw.png" alt="after switch" /></a></p>
  <p>my code for switching the theme:</p>
  <pre><code>    public class AppThemeService : IAppThemeService
      {
          public void ApplyTheme(ApplicationTheme? theme)
          {
              var elementTheme = theme.ToElementTheme();
              if (Window.Current.Content is FrameworkElement element)
                  element.RequestedTheme = elementTheme;
          }
      }
  </code></pre>
  <p>Any know a solution for this?</p>
  <p>Thanks.</p>