message: Update the tab colors some more
  Again, to reflect continuing discussion with the WinUI team. A doc may have been misread - these are the colors they're proposing, which are generally a lot better.
  See also:
  ![image](https://user-images.githubusercontent.com/18356694/157041174-c0eb2140-3b49-4494-8746-b3a12f396fe6.png)
diff: <!-- Define resources for Dark mode here -->
  <!--
  - This is the color of
  - ApplicationPageBackgroundThemeBrush, which is
  + Originally, we were using the raw value of
  + ApplicationPageBackgroundThemeBrush here, which is
  evaluated as SolidBackgroundFillColorBase. If we try
  to use those resources directly though, we don't get
  the properly themed versions. Presumably because the
  App itself can't have it's RequestedTheme changed at
  runtime.
  + However, after more discussion with the WinUI
  + team, we determined that those colors don't
  + provide enough contrast. The color here (and in
  + light mode) were chosen for greater contrast.
  +
  See GH #12356 for more history on the subject.
  -->
  <SolidColorBrush x:Key="TabViewBackground"
  - Color="#202020" />
  + Color="#0a0a0a" />
  <StaticResource x:Key="UnfocusedBorderBrush"
  ResourceKey="ApplicationPageBackgroundThemeBrush" />
  <!-- Define resources for Light mode here -->
  <!--
  - The Dark mode BG uses
  - ApplicationPageBackgroundThemeBrush, but we're
  - manually setting the Light BG to e8e8e8 here, at the
  - guidance of the WinUI team. Otherwise, there's just
  - not enough contrast in light mode. GH #12398
  + GH #12398 has more history on this value, as well as GH #12400
  -->
  <SolidColorBrush x:Key="TabViewBackground"
  - Color="#E8E8E8" />
  + Color="#dadada" />
  <StaticResource x:Key="UnfocusedBorderBrush"
  ResourceKey="ApplicationPageBackgroundThemeBrush" />
  