adjust header color for popup menu

[harrieshin]

Platforms Impacted

• iOS
• macOS

Description of changes

adjust the popupmenu headerfooterview background color to be clear to blend better with the overall drawercontroller

Binary change

Total increase: 2,200 bytes
Total decrease: 0 bytes

File	Before	After	Delta
Total	29,686,720 bytes	29,688,920 bytes	⚠️ 2,200 bytes

Full breakdown

File	Before	After	Delta
PopupMenuController.o	380,256 bytes	382,072 bytes	⚠️ 1,816 bytes
__.SYMDEF	4,497,496 bytes	4,497,880 bytes	⚠️ 384 bytes

Verification

Before	After

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: codeflow:open?pullrequest=#1833