To remove the transparent effect, you can use the ThemeEffectsHelper custom class and its attached property IsAcrylic like so:
ThemeEffectsHelper.IsAcrylicEnabled = false;
The transparent/blurred effect is only available on Windows 10. In any other operating system it would result in a transparent Window. You can read more on the Fluent article under Visual Effects Helpers.
Attached, you can find a sample project in which I've set the ThemeEffectsHelper.IsAcrylic property in MainWindow.xaml and set it commented in App.xaml.cs in case you want to use apply it globally.
I hope you find this helpful.
Regards,
Dimitar Dinev
Progress Telerik
Get quickly onboarded and successful with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers.Learn More.