Also very easy.
As you know (or not yet?) each menu item has it's own unqiue id class used in body class (just preview source code).

In above example (from your site) you can see "itemid-111" right, so you have to use
.itemid-111 #sp-main-body {padding-top: 0px;}
you can also (alternative) use
.view-article.itemid-121 #sp-main-body {padding-top: 0px;}

yes, both syntax are fully correct/working if correct item ID is used. It's how Joomla works, webmaster life easier if you know where to look :)
Unfortunately, there is no separate class for the selected category, which is a bit of a handicap.