fix(ThemeService): preserve 'System' theme preference on OS theme changes

[Illustar0]

Closes #2914

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

When the user set the application theme to AppTheme.System, the service would correctly apply ElementTheme.Default to follow the operating system's theme.

However, the first time the system theme changed (e.g., from light to dark), the ElementThemeChanged event handler would re-apply the new theme (AppTheme.Dark) as a permanent setting. This had two negative effects:

• It overwrote the user's preference: The saved setting was changed from AppTheme.System to AppTheme.Dark.
• It changed the active theme request: The RequestedTheme property was updated from ElementTheme.Default to the fixed ElementTheme.Dark.

As a result, the application would no longer respond to any future system theme changes, effectively "locking" the theme to whatever it first switched to.

What is the new behavior?

With the corrected logic, the ElementThemeChanged event handler behaves differently:

• It first checks if the user's saved preference is indeed AppTheme.System.
• If it is, the handler only fires a notification (ThemeChanged event) to inform other parts of the app that the visual theme has changed.
• Crucially, it does not call any methods to set or save the theme.

This ensures that the RequestedTheme property remains ElementTheme.Default and the user's AppTheme.System preference is preserved. The application now correctly follows all subsequent system theme changes without getting locked.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

Tested on the stable branch with Uno.SDK 6.3.28 + Uno.Extensions 6.2.3, and it works correctly.

[CLAassistant]

All committers have signed the CLA.

[Illustar0]

Hi @kazo0 ,

When you have a moment, could you please take a look at this PR?

[Copilot]

Pull Request Overview

This PR fixes a bug where the ThemeService would incorrectly overwrite a user's AppTheme.System preference when the operating system's theme changed, effectively "locking" the application to whichever theme it first switched to.

Key Changes:

• Refactored ElementThemeChanged to check the saved theme preference before reacting to OS theme changes
• The handler now only fires a notification event when the user preference is AppTheme.System, avoiding unintended theme persistence
• Removed the call to InternalSetThemeAsync from the event handler to preserve the user's original preference

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[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

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[Illustar0]

Tested again on the stable branch with Uno.SDK 6.3.28 + Uno.Extensions 6.2.3, and it works correctly.

[Illustar0]

@kazo0

[Illustar0]

@kazo0 Thanks for the approval! Could you please merge this?