@kayleerichard Here is the code you can use to get the desired highlight if actual completion date is greater than the planned completion date.
You'll want to navigate to the column you want to highlight, switch to text mode and paste this below the data that populates.
styledef.case.0.comparison.icon=false
styledef.case.0.comparison.leftmethod=actualCompletionDate
styledef.case.0.comparison.lefttext=actualCompletionDate
styledef.case.0.comparison.operator=gt
styledef.case.0.comparison.operatortype=date
styledef.case.0.comparison.rightmethod=plannedCompletionDate
styledef.case.0.comparison.righttext=plannedCompletionDate
styledef.case.0.comparison.trueproperty.0.name=bgcolor
styledef.case.0.comparison.trueproperty.0.value=eac6c9
styledef.case.0.comparison.truetext=
Also, the link Skye posted is great for learning how to do this.