If the XAML Designer manages to display the viewer with its theme, but at run-time the theme is not applied, this must be an issue with how XAML resources are loaded.
Please check the paths to the XAML files, and if you use relative paths, adjust them based on the application's execution folder or copy the XAML resources relatively to the execution folder.
If you use the suggested URI, it should be considered with the project in which XAML resources are built-in - the name of the project and the folder with XAML resources.
In order to provide you more accurate suggestions, please open a support ticket and send us a demo project that illustrates the settings and reproduces the problem.
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