Fixes DataGrid colors not updating on theme switch (#4138)

## Fixes #4119
Fixes DataGrid colors not updating on theme switch.


## PR Type
What kind of change does this PR introduce?
- Bugfix


## What is the current behavior?
Some DataGrid row group header brushes do not update upon theme switching, resulting in a state where rows are in a theme and row group headers are in the opposite theme. This behavior is undesirable.


## What is the new behavior?
In this update the related brushes were moved into the ThemeDictionaries.
After these changes, the DataGrid as a whole switches from a theme to another.


## PR Checklist

Please check if your PR fulfills the following requirements:

- [ ] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [ ] Sample in sample app has been added / updated (for bug fixes / features)
    - [ ] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/CommunityToolkit/WindowsCommunityToolkit-design-assets)
- [ ] New major technical changes in the toolkit have or will be added to the [Wiki](https://github.com/CommunityToolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
- [ ] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [ ] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes


## Other information
Tested in Windows 21H1 only.

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.UI.Controls.DataGrid/DataGrid/DataGrid.xaml

@@ -28,6 +28,10 @@
             <StaticResource x:Key="DataGridColumnHeaderDraggedBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPointerOverBrush" ResourceKey="SystemControlHighlightListLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPressedBrush" ResourceKey="SystemControlHighlightListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderPressedBackgroundBrush" ResourceKey="SystemControlBackgroundListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderForegroundBrush" ResourceKey="SystemControlForegroundBaseHighBrush"/>
+            <StaticResource x:Key="DataGridRowHoveredBackgroundColor" ResourceKey="SystemListLowColor"/>
         </ResourceDictionary>
         <ResourceDictionary x:Key="HighContrast">
             <SolidColorBrush x:Key="InvalidBrush" Color="#FFFF00"/>
@@ -41,6 +45,10 @@
             <StaticResource x:Key="DataGridColumnHeaderDraggedBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPointerOverBrush" ResourceKey="SystemControlHighlightListLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPressedBrush" ResourceKey="SystemControlHighlightListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderPressedBackgroundBrush" ResourceKey="SystemControlBackgroundListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderForegroundBrush" ResourceKey="SystemControlForegroundBaseHighBrush"/>
+            <StaticResource x:Key="DataGridRowHoveredBackgroundColor" ResourceKey="SystemListLowColor"/>
         </ResourceDictionary>
         <ResourceDictionary x:Key="Light">
             <SolidColorBrush x:Key="InvalidBrush" Color="#C50500"/>
@@ -54,6 +62,10 @@
             <StaticResource x:Key="DataGridColumnHeaderDraggedBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPointerOverBrush" ResourceKey="SystemControlHighlightListLowBrush"/>
             <StaticResource x:Key="DataGridColumnHeaderPressedBrush" ResourceKey="SystemControlHighlightListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderPressedBackgroundBrush" ResourceKey="SystemControlBackgroundListMediumBrush"/>
+            <StaticResource x:Key="DataGridRowGroupHeaderForegroundBrush" ResourceKey="SystemControlForegroundBaseHighBrush"/>
+            <StaticResource x:Key="DataGridRowHoveredBackgroundColor" ResourceKey="SystemListLowColor"/>
         </ResourceDictionary>
     </ResourceDictionary.ThemeDictionaries>
 
@@ -89,13 +101,9 @@
     <StaticResource x:Key="DataGridRowSelectedUnfocusedBackgroundOpacity" ResourceKey="ListAccentLowOpacity"/>
     <StaticResource x:Key="DataGridRowSelectedHoveredUnfocusedBackgroundColor" ResourceKey="SystemAccentColor"/>
     <StaticResource x:Key="DataGridRowSelectedHoveredUnfocusedBackgroundOpacity" ResourceKey="ListAccentMediumOpacity"/>
-    <StaticResource x:Key="DataGridRowHoveredBackgroundColor" ResourceKey="SystemListLowColor"/>
     <StaticResource x:Key="DataGridRowHeaderForegroundBrush" ResourceKey="SystemControlForegroundBaseMediumBrush"/>
     <StaticResource x:Key="DataGridRowHeaderBackgroundBrush" ResourceKey="SystemControlBackgroundAltHighBrush"/>
-    <StaticResource x:Key="DataGridRowGroupHeaderBackgroundBrush" ResourceKey="SystemControlBackgroundChromeMediumBrush"/>
     <StaticResource x:Key="DataGridRowGroupHeaderHoveredBackgroundBrush" ResourceKey="SystemControlBackgroundListLowBrush"/>
-    <StaticResource x:Key="DataGridRowGroupHeaderPressedBackgroundBrush" ResourceKey="SystemControlBackgroundListMediumBrush"/>
-    <StaticResource x:Key="DataGridRowGroupHeaderForegroundBrush" ResourceKey="SystemControlForegroundBaseHighBrush"/>
     <StaticResource x:Key="DataGridRowInvalidBrush" ResourceKey="InvalidBrush"/>
     <StaticResource x:Key="DataGridCellBackgroundBrush" ResourceKey="SystemControlTransparentBrush"/>
     <StaticResource x:Key="DataGridCellFocusVisualPrimaryBrush" ResourceKey="SystemControlFocusVisualPrimaryBrush"/>
@@ -967,4 +975,4 @@
             </Setter.Value>
         </Setter>
     </Style>
-</ResourceDictionary>
+</ResourceDictionary>

Microsoft.Toolkit.Uwp.UI.Controls.DataGrid/DataGrid/DataGridRow.cs

@@ -1122,40 +1122,11 @@ internal void EnsureForeground()
             {
                 DiagnosticsDebug.Assert(this.Index != -1, "Expected Index other than -1.");
 
-                PropertyMetadata metadataInfo = DataGridRow.ForegroundProperty.GetMetadata(typeof(DataGridRow));
-                Brush defaultForeground = metadataInfo == null ? null : metadataInfo.DefaultValue as Brush;
-                Brush newForeground = null;
+                var newForeground = this.Index % 2 == 0 || this.OwningGrid.AlternatingRowForeground == null
+                    ? this.OwningGrid.RowForeground
+                    : this.OwningGrid.AlternatingRowForeground;
 
-                if (this.Foreground.Equals(defaultForeground))
-                {
-                    if (this.Index % 2 == 0 || this.OwningGrid.AlternatingRowForeground == null)
-                    {
-                        // Use OwningGrid.RowForeground if the index is even or if the OwningGrid.AlternatingRowForeground is null
-                        if (this.OwningGrid.RowForeground != null)
-                        {
-                            newForeground = this.OwningGrid.RowForeground;
-                        }
-                    }
-                    else
-                    {
-                        // Alternate row
-                        if (this.OwningGrid.AlternatingRowForeground != null)
-                        {
-                            newForeground = this.OwningGrid.AlternatingRowForeground;
-                        }
-                    }
-
-                    if (newForeground == null)
-                    {
-                        newForeground = this.Foreground;
-                    }
-                }
-                else
-                {
-                    newForeground = this.Foreground;
-                }
-
-                this.ComputedForeground = newForeground;
+                this.ComputedForeground = newForeground ?? this.Foreground;
             }
             else
             {