访问控制模板中的数据模板元素

I have a ListBox with an ItemTemplateSelector. All the data templates returned by the template selector have a Border. I'd like to change the border's background color when the mouse is over it. To achieve this, I added to the listbox a ListBoxItem control template with a trigger for IsMouseOver but I have no idea how to reference the data template's Border from the trigger's setter. My best bet was the following setter but it had no effect:I have a ListBox with an ItemTemplateSelector.