Thank you for the provided picture.
The System.Windows.xaml ResourceDictionary contains styles for some of native WPF controls. If you merge this resource, provided that you do not set a style with a higher priority, those controls will receive an implicit style from that ResourceDictionary. You can open the System.Windows.xaml file for the theme that you are using from the Themes.Implicit folder in your UI for WPF installation directory to see all the styles that are within that ResourceDictionary.
If you don't want the native WPF controls' styles to be altered, you can simply avoid merging the System.Windows.xaml ResourceDictionary. This will not cause any side effects for the UI for WPF controls.
I hope you find this helpful.
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