Fix insufficient contrast for PropertyGrid dropdown text in Dark Mode

[LeafShi1]

Fixes #14120

Root Cause

The DesignBindingPicker (used for DataSource/DisplayMember property editors) were applying the Explorer theme regardless of the color mode. In Dark Mode, this caused insufficient contrast for the None text and other items, especially when selected.

Proposed changes

• Update DesignerUtils.cs: Modified ApplyTreeViewThemeStyles method to apply DarkMode_Explorer theme when Application.IsDarkModeEnabled is true, otherwise use Explorer theme

Customer Impact

• Users working with Dark Mode enabled will now experience significantly improved readability when using PropertyGrid dropdowns for DataSource, DisplayMember

Regression?

• No

Risk

• Minimal

Screenshots

Before

Insufficient contrast for the "None" text in the Dropdown Panel for the DataSource/DisplayMemeber properties in the DarkMode

After

The contrast of the text "None" is sufficient.

Test methodology

• Perform manual testing using the Accessibility Insight tool.

Test environment(s)

• .net 11.0.0-alpha.1.25617.103

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.15493%. Comparing base (01eefd4) to head (919d2d5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14152         +/-   ##
===================================================
+ Coverage   77.15180%   77.15493%   +0.00312%     
===================================================
  Files           3279        3279                 
  Lines         645333      645334          +1     
  Branches       47720       47721          +1     
===================================================
+ Hits          497886      497907         +21     
+ Misses        143754      143740         -14     
+ Partials        3693        3687          -6     

Flag	Coverage Δ
Debug	77.15493% <50.00000%> (+0.00312%)	⬆️
integration	18.99566% <0.00000%> (+0.01559%)	⬆️
production	52.02118% <50.00000%> (+0.00711%)	⬆️
test	97.40559% <ø> (ø)
unit	49.46171% <50.00000%> (-0.00574%)	⬇️

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