Update system-backdrop-controller.md - Add a note that materials with customized properties do not react to theme changes

[MuyuanMS]

Add a note on materials with customized controller properties will no longer reacts to Light/Dark theme changes.

Doc change request based on community reported issue for Windows App SDK: microsoft/WindowsAppSDK#3589 and microsoft/WindowsAppSDK#3589, where the developer was confused when, after flipping theme between Dark and Light, materials with non-default controller properties fail to automatically make corresponding changes.

[prmerger-automator]

@MuyuanMS : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit a8e9fa7:

💡 Validation status: suggestions

File	Status	Preview URL	Details
hub/apps/windows-app-sdk/system-backdrop-controller.md	💡Suggestion		Details

hub/apps/windows-app-sdk/system-backdrop-controller.md

• Line 220, Column 132: [Suggestion: preserve-view-not-set - See documentation] You've pinned this link to a specific version of content with the view parameter. It's recommended not to pin a version unless that version is A) not the default view and B) the context is about that version specifically. To proceed with pinning a version add the &preserve-view=true to the URL. Otherwise, remove the view parameter. URL: /windows/windows-app-sdk/api/winrt/microsoft.ui.composition.systembackdrops.systembackdropconfiguration.theme?view=windows-app-sdk-1.6#microsoft-ui-composition-systembackdrops-systembackdropconfiguration-theme

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 0eac54f:

✅ Validation status: passed

File	Status	Preview URL	Details
hub/apps/windows-app-sdk/system-backdrop-controller.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel