Styling the indeterminate state of a WPF checkbox

Question

I want to style the indeterminate state of a WPF checkbox. We have a treeview control with checkboxes, and we want the indeterminate state to represent that some descendants are

Answer 1

You can set the checkbox's IsThreeState property to true.

This, however, allows toggling the checkbox's value to null.
If that is undesired, you can instead add to your CheckBox's Template a trigger for the null value, just like Greg Bacchus shows in his answer: