I actually found the code in header.php
<?php
if(trim($options['sidebarwidth']) == "") $options['sidebarwidth'] = 270;
$sidebarwidth = intval($options['sidebarwidth']);
$containerwidth = 860 - $sidebarwidth;
?>
860 was based on the 900px wide (40 px fo right and left padding), so by adding a 100 to 860, that gap is gone and I have a larger content now (960px – sidebarwidth)