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

Using the template from Blend:

To make BulletChrome work, you need to add a reference to PresentationFramework.Aero, and add the xml namespace declaration for the "theme" namespace:

xmlns:theme="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero"

I didn't try this myself but I believe it should work (I've done it with Luna).