If you want the main background to show thru, set the background color to 'transparend' in all the locations that cover the background (i.e. post styling, center styling etc) If you want to make the center area have an opacity, you can do the following
Go to ATO->Style & edit CENTER COLUMN and in the section 'Content ABOVE the LOOP' add the following at the very end
HTML Code:
<div id='mycenter'>
next go to the section 'Content BELOW the LOOP' and add the following AS THE FIRST LINE
HTML Code:
<div id='mycenter'>
next go to ATO->Add HTML/CSS Inserts->CSS Inserts and add the following
HTML Code:
div#mycenter {
filter:alpha(opacity=60);
-moz-opacity:.60;
opacity:0.6;
}