How do you create a feature outline that becomes increasingly transparent from the outside to the inside of the outline?

[BradSmith]

I have seen maps where a feature has a thick outline that is darker around the outside edge and gradually becomes lighter or more transparent toward the inside of the outline. Does anyone know how to do this?

Here is an example of what I am talking about.

polygon symbol‌ #symbology

[EdiePunt]

Hi Brad. Set the fill symbol layer in the polygon symbol to gradient fill.