A ControlTemplate specifies the visual structure and visual behavior of a control. You can customize the appearance of a control by giving it a new ControlTemplate. When you create a ControlTemplate, you replace the appearance of an existing control without changing its functionality.
For example, you can make the buttons in your application round rather than the default square shape, but the button will still raise the Click event. As you create a ControlTemplate in XAML, you can change a control's appearance without writing any code.
Read - Walkthrough: Customizing the Appearance of a Button by Using a ControlTemplate
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