After you add that to your CSS, you will also need to define a class for the main table with a white background and then add THAT to the CSS as well.
For instance, in the rectangle_top.tpl template, add
to the <Table> definition also setting the table width to some fixed pixel ammount - so it reads something like:
Code:
<TABLE align="center" border="0" cellpadding="0" cellspacing="0" width="768" height="100%" class="MainTable">
Then in the CSS add the class definition
Code:
.MainTable {
background-color: #FFFFFF;
}