Fixed DataGrid column header text color in HC mode

[dipeshmsft]

Description

In HC mode, the DataGrid column header text color was not visible. DataGrid column header was inheriting foreground color from the DataGrid control. This PR introduces a new resource DataGridColumnHeaderForeground color which is now used as the color of the column header text.

Customer Impact

Customers using Desert mode won't be able to see the column headers correctly.

Regression

Yes. Introduced in .NET 10 Preview 5 / 6.

Testing

Local app testing.

Risk

Minimal

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR fixes low visibility of DataGrid column header text in high contrast mode by introducing a new resource, DataGridColumnHeaderForeground, and using it in multiple theme files.

• Introduces a new SolidColorBrush resource for DataGridColumnHeaderForeground in several theme files.
• Applies the new resource via a Setter in the DefaultDataGridColumnHeaderStyle in each theme.
• Updates both HC and conventional themes to use the new color resource ensuring consistent visibility across modes.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Fluent.xaml	Adds a Foreground Setter referencing the new resource in the default DataGridColumnHeader style.
Fluent.Light.xaml	Introduces the DataGridColumnHeaderForeground brush and applies it via the style.
Fluent.HC.xaml	Defines the HC-specific DataGridColumnHeaderForeground brush and updates the header style accordingly.
Fluent.Dark.xaml	Adds the new Foreground Setter and brush for Dark theme.
DataGrid.xaml	Updates the DataGrid header style to use the dynamic resource for Foreground.
Theme/Light.xaml	Adds the brush definition for DataGridColumnHeaderForeground in Light theme resources.
Theme/HC.xaml	Updates HC theme resources with the new brush definition.
Theme/Dark.xaml	Inserts the new brush definition for Dark theme resources.

Comments suppressed due to low confidence (2)

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Styles/DataGrid.xaml:407

• Add tests to verify that the DataGrid column header renders with the correct foreground color in both normal and high contrast modes. This will help ensure that the fix consistently improves header visibility across themes.

        <Setter Property="Foreground" Value="{DynamicResource DataGridColumnHeaderForeground}" />

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Themes/Fluent.HC.xaml:373

• Ensure that the SystemColorButtonTextColor used for DataGridColumnHeaderForeground in HC mode provides sufficient contrast for accessibility guidelines. Consider validating this color choice against accessibility standards.

  <SolidColorBrush x:Key="DataGridColumnHeaderForeground" Color="{StaticResource SystemColorButtonTextColor}" />

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 13.45748%. Comparing base (28843aa) to head (871759f).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10956         +/-   ##
===================================================
- Coverage   13.52892%   13.45748%   -0.07144%     
===================================================
  Files           3319        3319                 
  Lines         664894      664894                 
  Branches       74674       74674                 
===================================================
- Hits           89953       89478        -475     
- Misses        572398      572864        +466     
- Partials        2543        2552          +9     

Flag	Coverage Δ
Debug	13.45748% <ø> (-0.07144%)	⬇️

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