Igor is correct, the best way to learn mojoPortal CSS classes is to study the page source in your browser. I, and many others, use FireBug for this because it makes viewing the markup and CSS rules a lot easier.
I took a quick look at your site and as far as the padding and margins go for the modules, there is a 5px margin on the art-Post class and a 10px padding on the art-Post-body class. What this does is put a total of 30px between the modules. If you set the margins and paddings to only be on the top and not the bottom, you would drop the space from 30px to 15px between the modules.