message: Remove the 1px gap under inactive tabs
  As noted in
  Screenshot below.
  * [x] Closes a single element of
diff: <!-- Suppress top padding -->
  <Thickness x:Key="TabViewHeaderPadding">9,0,5,0</Thickness>
  + <Thickness x:Key="TabViewItemBorderThickness">1,1,1,0</Thickness>
  <!-- Shadow that can be used by any control. -->
  <ThemeShadow x:Name="SharedShadow" />
  