If you set OverridesDefaultStyle to true on a control, you will be suppressing the default control template supplied by the theme styles. That control template typically includes the content presenter and other composited elements that provide basic UI functionality and visualization for the control. If you want the control to continue to support the same features as the default theme styles, you must supply an alternate style with a control template that replicates the same structure. For more information, see Control Authoring Overview.

[http://msdn.microsoft.com/en-us/library/system.windows.frameworkelement.overridesdefaultstyle.aspx]