Fix runtime theme resource replacement

[jfversluis]

Summary

• replace MergedDictionaries.Clear() with removal of only the active theme dictionary
• preserve app-level dictionaries such as Colors.xaml and Styles.xaml
• instantiate the default theme as a typed dictionary so it can be identified at runtime

Fixes #3468

Verification

The current .NET MAUI app template merges Colors.xaml and Styles.xaml into Application.Resources, so clearing MergedDictionaries removes resources unrelated to the selected theme. The updated example removes only LightTheme or DarkTheme before adding the selected theme.

Upstream references:

• Current .NET MAUI app template resources
• Resource dictionary removal test coverage

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Internal previews

File	Preview link
docs/user-interface/theming.md	docs/user-interface/theming

[learn-build-service-prod]

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[learn-build-service-prod]

Learn Build status updates of commit dc03b4c:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/user-interface/theming.md	✅Succeeded	View (>=net-maui-8.0)

For more details, please refer to the build report.

[Copilot]

Pull request overview

Updates the theming documentation example to avoid clearing all merged resource dictionaries when switching themes at runtime, preventing removal of app-template resources (for example, Colors.xaml and Styles.xaml).

Changes:

• Update the default theme XAML example to instantiate the default theme as a typed dictionary (so it can be identified later).
• Update runtime theme-switching guidance to remove only the active theme dictionary instead of clearing all merged dictionaries.
• Refresh article metadata date.

Show a summary per file

File	Description
docs/user-interface/theming.md	Updates theme resource loading examples to remove only the active theme dictionary and preserve other app-level merged dictionaries.

Review details

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• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite