message: Give our NavigationView's acrylic a fallback color
  It will be a different color than the background, so it will look less
  weird when it's unfocused. It also fixes the bug where the navigation
  menu is transparent when acrylic is disabled systemwide.
  Fixes
diff: <AcrylicBrush x:Key="NavigationViewDefaultPaneBackground"
  BackgroundSource="Backdrop"
  + FallbackColor="{ThemeResource SystemChromeMediumLowColor}"
  TintColor="{ThemeResource SystemChromeMediumColor}"
  TintOpacity="0.5" />
  <AcrylicBrush x:Key="NavigationViewTopPaneBackground"
  BackgroundSource="Backdrop"
  + FallbackColor="{ThemeResource SystemChromeMediumLowColor}"
  TintColor="{ThemeResource SystemChromeMediumColor}"
  TintOpacity="0.5" />
  <AcrylicBrush x:Key="NavigationViewExpandedPaneBackground"
  BackgroundSource="HostBackdrop"
  + FallbackColor="{ThemeResource SystemChromeMediumLowColor}"
  TintColor="{ThemeResource SystemChromeMediumColor}"
  TintOpacity="0.7" />
  </ResourceDictionary>
  