How do I change the margins

Please could someone help with their expert skills.

I have a Volusion website, www.dazzle.uk.com. There was a left nav on each of the sub category pages which has been removed. There is now remaining a black space on each of the sub category pages - see example when clicking here http://www.dazzle.uk.com/Childrens_Party_Supplies_s/402.htm

All I want to do is remove the margin so that the space is utilised.

I was told to add into the site content page
/* --- OVERRIDES --- */

#content {margin-top: 9px;}
#leftnav {display: none;}
#topnav {margin-bottom: 10px;}
#content_area {
width: 950px;
float: none;
margin: 0 auto;
}

but nothing has changed.

Please can someone help asap
Thank you