XF 1.4 Change thread / discussion list border lines

I'd like to change the color/thickness of the lines below each thread / discussion title (see screenshot below which shows the thin line between thread titles).

I couldn't find a way to do it in Style Properties.


Screen Shot 2015-10-29 at 10.57.17 PM.jpg
Thanks Brogan. I've referenced that guide but still cannot come up with a solution. I know the template is thread_list_item but I cannot locate any css which controls the appearance of the line between thread titles. And don't see anything for it in style properties.

Anyone have a lead? Or a suggestion on how to change the line's look in extra.css? Thx.