Fix the SUI being in the wrong Theme color (#13145)

ThemeResources are a persistent pain.

Regressed in #13083. See also #12775 et. al.

We can't just put those here though as StaticResources, because XAML will evaluate their values when the App is first loaded, and we'll always use the value from the OS theme, regarless of the requested theme. Kinda the same thing we've had to do with TabViewBackground in the past.

* [x] Fixes something we noticed right before shipping

(cherry picked from commit bb03b00)
Service-Card-Id: 82303611
Service-Version: 1.13

src/cascadia/TerminalApp/App.xaml

@@ -153,10 +153,6 @@
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />
-
-                            <StaticResource x:Key="SettingsPageBackground"
-                                            ResourceKey="SolidBackgroundFillColorTertiary" />
-
                         </ResourceDictionary>
 
                         <ResourceDictionary x:Key="Light">
@@ -170,9 +166,6 @@
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />
-
-                            <StaticResource x:Key="SettingsPageBackground"
-                                            ResourceKey="SolidBackgroundFillColorTertiary" />
                         </ResourceDictionary>
 
                         <ResourceDictionary x:Key="HighContrast">
@@ -190,9 +183,6 @@
                             -->
                             <StaticResource x:Key="TabViewBackground"
                                             ResourceKey="SystemColorButtonFaceColorBrush" />
-
-                            <StaticResource x:Key="SettingsPageBackground"
-                                            ResourceKey="SystemColorWindowColorBrush" />
                         </ResourceDictionary>
 
                     </ResourceDictionary.ThemeDictionaries>

src/cascadia/TerminalSettingsEditor/MainPage.xaml

@@ -9,13 +9,43 @@
       xmlns:local="using:Microsoft.Terminal.Settings.Editor"
       xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
       xmlns:muxc="using:Microsoft.UI.Xaml.Controls"
-      Background="{ThemeResource SettingsPageBackground}"
       mc:Ignorable="d">
 
     <Page.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>
                 <ResourceDictionary Source="CommonResources.xaml" />
+
+                <ResourceDictionary>
+                    <ResourceDictionary.ThemeDictionaries>
+
+                        <!--
+                            These two values are "SolidBackgroundFillColorTertiary".
+                            We can't just put those here though as
+                            StaticResources, because XAML will evaluate their
+                            values when the App is first loaded, and we'll
+                            always use the value from the OS theme, regardless
+                            of the requested theme. Kinda the same thing we've
+                            had to do with TabViewBackground in the past.
+                        -->
+
+                        <ResourceDictionary x:Key="Dark">
+                            <Color x:Key="SettingsPageBackground">#282828</Color>
+                        </ResourceDictionary>
+
+                        <ResourceDictionary x:Key="Light">
+                            <Color x:Key="SettingsPageBackground">#F9F9F9</Color>
+                        </ResourceDictionary>
+
+                        <ResourceDictionary x:Key="HighContrast">
+                            <!--  Define resources for HighContrast mode here  -->
+                            <StaticResource x:Key="SettingsPageBackground"
+                                            ResourceKey="SystemColorWindowColorBrush" />
+                        </ResourceDictionary>
+
+                    </ResourceDictionary.ThemeDictionaries>
+                </ResourceDictionary>
+
             </ResourceDictionary.MergedDictionaries>
 
             <SolidColorBrush x:Key="NavigationViewExpandedPaneBackground"
@@ -30,7 +60,7 @@
     </Page.Resources>
 
     <muxc:NavigationView x:Name="SettingsNav"
-                         Background="Transparent"
+                         Background="{ThemeResource SettingsPageBackground}"
                          IsBackButtonVisible="Collapsed"
                          IsSettingsVisible="False"
                          ItemInvoked="SettingsNav_ItemInvoked"