Manually set the colors of the TabViewBackground (#12460)

This has been a saga.

Basically, any resources in `App.xaml` aren't going to be able to reference other theme-aware resources. We can't change the theme of the app at runtime, only elements within the app. So we can't use `ApplicationPageBackgroundThemeBrush` in app.xaml, because it will ALWAYS be evaluated as the OS theme version of that brush.

* regressed in #12326
* See also #10864
* #3917 CANNOT be fixed in the same way. We're lucky here that the TabView uses a `{ThemeResource TabViewBackground}` in markup to set the bg. We're not similarly lucky with the Pane one. 
* [x] closes #12356
* [x] Tested manually. You can confirm, my eyes are bleeding from the OS-wide light mode

src/cascadia/TerminalApp/App.xaml

@@ -54,17 +54,31 @@
                     <ResourceDictionary.ThemeDictionaries>
                         <ResourceDictionary x:Key="Dark">
                             <!--  Define resources for Dark mode here  -->
-                            <StaticResource x:Key="TabViewBackground"
-                                            ResourceKey="ApplicationPageBackgroundThemeBrush" />
+
+                            <!--
+                                This is the color of
+                                ApplicationPageBackgroundThemeBrush, 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.
+                                
+                                See GH #12356 for more history on the subject.
+                            -->
+                            <SolidColorBrush x:Key="TabViewBackground"
+                                             Color="#202020" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />
+
+
                         </ResourceDictionary>
 
                         <ResourceDictionary x:Key="Light">
                             <!--  Define resources for Light mode here  -->
-                            <StaticResource x:Key="TabViewBackground"
-                                            ResourceKey="ApplicationPageBackgroundThemeBrush" />
+                            <SolidColorBrush x:Key="TabViewBackground"
+                                             Color="#F3F3F3" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -156,7 +156,6 @@ namespace winrt::TerminalApp::implementation
         if (_settings.GlobalSettings().UseAcrylicInTabRow())
         {
             const auto res = Application::Current().Resources();
-
             const auto lightKey = winrt::box_value(L"Light");
             const auto darkKey = winrt::box_value(L"Dark");
             const auto tabViewBackgroundKey = winrt::box_value(L"TabViewBackground");