WPF XAML Checkbox: Trigger OnFocus to Color Background inside Checkbox square ONLY

I've been looking for several hours and cannot find how to do this, so forgive if it's already out there...

I'm looking for a way to code an OnFocus change of the background color of a Checkbox using XAML... changing the color of ONLY THE INSIDE OF THE CheckBox BOX itself, NOT the background of the text content alongside it. When focus is lost, go back to the "normal" color.

Thanks in advance to anyone who can help.