Hello,
You can change the bottom margin here, just remove the padding bottom and change to margin and this will decrease white space but increase space below module:
#s5_tabshow_left {
background: none repeat scroll 0 0 #FFFFFF;
box-shadow: 0 1px 2px 2px rgba(0, 0, 0, 0.1);
margin-bottom: 25px;
}
http://www.asafl.is/modules/mod_s5_tabshow/s5_tabshow/styles.css (line 156)
As for other changes I would just use firebug and use the "inspect" tool and hover over items on your site and then you can see what CSS is formatting them. We have a tutorial for firebug located here too:
http://www.shape5.com/joomla_tutorials.html