触发器中的WPF ControlTemplate样式GradientStop

Here is my XAML for a TabItem. I want to be able to set the Color of a single gradient stop in a trigger. I know that I can re-define the gradient completely in the trigger's setter, but I want to access a specific property on the background so I can animate it in the future.Here is my XAML for a TabItem. I want to be abl