Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

A few changes to improve the experience of using Alias Tokens on macOS.

• Create a new PlatformThemeProviding protocol to let macOS supply its own alias token values
• Move visionOS to use this new protocol for its existing overrides
• Move some platform-agnostic ViewModifier functions into FluentUI_common
• Create a new AliasColorTokens demo view
• Add visionOS support to the new Demo app

Verification

Verified that override colors are implemented correctly on macOS and visionOS.

Visual Verification

Dark	Light
	n/a

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: Open in CodeFlow