XF 1.3 Sub-Forum list displayed vertically?


I have a forum that has 10 sub-forums. These are set so as to be displayed as a list below the parent node. At the moment there are displaying as a list horizontally across the page. I would like them to display as a vertical list below the parent node. I presume this is possible and that I have missed a setting somewhere. Could someone please explain how to change it to a vertical list?

Thanks for your reply Andy. The vertical list displayed in the pop up is exactly what I want but without the pop-up. I want the sub-forum list to permanently display as a vertical list below the parent node without having to use the pop-up to display the list. Any ideas? :)
Thanks Sheldon, that is exactly what I require. Being a total noob, where exactly do I copy/paste that CSS code if you don't mind me asking? :)