message: Update light theme tab background for greater contrast.
  Changes the tab view BG to `#e8e8e8`, as discussed in mail thread.
  Closes
diff: <ResourceDictionary x:Key="Light">
  <!-- 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
  + -->
  <SolidColorBrush x:Key="TabViewBackground"
  - Color="#F3F3F3" />
  + Color="#E8E8E8" />
  <StaticResource x:Key="UnfocusedBorderBrush"
  ResourceKey="ApplicationPageBackgroundThemeBrush" />
  