XAML Trigger for Default Control Highlight

This XAML code defines a trigger for a control's 'IsDefaulted' property. When the 'IsDefaulted' property is set to true, the trigger sets the 'BorderBrush' property of the control's 'border' element to a dynamic resource that resolves to the system highlight brush. This is typically used to visually indicate that the control is the default control in a group of controls.