I'm not sure I understand what you're asking for, and others are probably confused by your terminology. "space between forums" is meaningless... an SMF installation is a single forum, divided up into categories, which are divided up into boards, which are divided up into topics, which are divided up into posts.
From your pictures, I'm guessing that you want some extra space between boards. That could probably be done by adding some margin-bottom to the CSS, if you want the background showing through. If you want something other than the background to show through, you'll probably have to change code to insert a new line of text, just a with a very tiny font size and a specified background. You might have to indent this line a bit if the theme you use has rounded corners on the boards, and make its <div> shorter, so that it doesn't stick out into where the rounded corners are and look ugly.
The theme used here already has a little bit of gap (with background) between boards. Is that the effect you're looking for? You might look at the HTML source for the page and the CSS used and figure out from there if they added some margin or stuck in a dummy line.