The ControlTemplate specifies the appearance of a Control; if a Control does not have a ControlTemplate, the Control will not appear in your application. The control author defines the default control template, and the application author can override the ControlTemplate to redefine the visual tree of the control. See ControlTemplate Examples for information and examples of how to change the visual tree of existing controls.
A ControlTemplate is intended to be a self-contained unit of implementation detail that is invisible to outside users and objects, including Style objects. The only way to manipulate the content of the control template is from within the same control template.