I need to put a single line border between my products listed on each category page. I'm using "entity.simpleproductlist.xml.config"

My "Specials" product list has a line between each product by default. I just want to do the same thing for the list of products on category pages.

I've tried editing style.css and "entity.simpleproductlist.xml.config" but can't get a line to display between products.