来自Style DataTrigger的TemplateBinding在ControlTemplate中

In the following XAML I'm using a Rectangle with a Border as the Template for a ToggleButton. I want the BorderBrush to be a different colour to reflect the changing value of ToggleButton.IsChecked. Unfortunately my attempt here of using a TemplateBinding in the DataTrigger doesn't work. What do I need to do instead?In the following XAML I'm using a Rectangle wit