Ok I see it now. Yeah it looks like they added some custom code in order to get that working in the way that it is. I thought you meant something that would require a small CSS tweak (like a border).
In order to get that same feature it would take some custom code/development work to get it setup like that. They may have coded a custom template for their posts to look like that. That site uses Genesis, so that means it’s definitely possible with this theme, it would take some time to figure it out though.