Fix Settings page text not adopting High Contrast Aquatic/Desert themes (#2463)

Add HighContrast theme dictionary to Settings.xaml that overrides
TextFillColorPrimaryBrush and TextFillColorSecondaryBrush to use
SystemColorWindowTextColor. Also fix SettingsRichTextBlockStyle using
TextFillColorPrimary (a Color) instead of TextFillColorPrimaryBrush
(a Brush) for the Foreground property.

Author: emmche

src/Calculator/Views/Settings.xaml

@@ -13,6 +13,13 @@
 
     <UserControl.Resources>
         <ResourceDictionary>
+            <ResourceDictionary.ThemeDictionaries>
+                <ResourceDictionary x:Key="HighContrast">
+                    <SolidColorBrush x:Key="TextFillColorPrimaryBrush" Color="{ThemeResource SystemColorWindowTextColor}"/>
+                    <SolidColorBrush x:Key="TextFillColorSecondaryBrush" Color="{ThemeResource SystemColorWindowTextColor}"/>
+                </ResourceDictionary>
+            </ResourceDictionary.ThemeDictionaries>
+
             <Style x:Key="SettingsContentScrollViewStyle" TargetType="ScrollViewer">
                 <Setter Property="HorizontalAlignment" Value="Stretch"/>
                 <Setter Property="VerticalAlignment" Value="Stretch"/>
@@ -27,7 +34,7 @@
 
             <Style x:Key="SettingsRichTextBlockStyle" TargetType="RichTextBlock">
                 <Setter Property="HorizontalAlignment" Value="Left"/>
-                <Setter Property="Foreground" Value="{ThemeResource TextFillColorPrimary}"/>
+                <Setter Property="Foreground" Value="{ThemeResource TextFillColorPrimaryBrush}"/>
                 <Setter Property="FontSize" Value="{ThemeResource BodyFontSize}"/>
                 <Setter Property="TextWrapping" Value="Wrap"/>
             </Style>