CSS and PHP templates

OK I pretty much have this down for Joomla and Drupal but Zencart is throwing me for a loop. Their default template (they use an override system) has so many classes and ids that it is hard to work around..

I have found myself here....

http://bjrtechnologies.com/zen/

If you look at the source and around all their includes. I am trying to use just a few main divisions...

body {
margin : 0;
padding : 0;
text-align : center;
font-family : verdana, arial, sans-serif;
font-size : 11px;
}
.centeredContent {
text-align : center;
}
.back {
float : left;
}
.clearBoth {
clear : both;
}
#mainWrapper {
border : 1px solid black;
width : 950px;
}
#header {
height : 250px;
background: yellow;
}
#contentMainWrapper {
width : 950px;
background: pink;
}
#middle {
width : 650px;
float : left;
}
#navColumnOne {
background : blue;
float : left;
}
#navColumnTwo {
background : blue;
float : left;
}

	
	#footer {
		background: pink;
	}

Code (markup):

however in Netscape you cannot see the background of
#contentMainWrapper


I have found that if this occurs I must have missed a clear float but I cannot find it....please any advice would stop the insanity.