With proSilve IP Blue you have to edit both parts #global-wrapper and /* wrapper */ within the style_blue.css
#global-wrapper {
width: 75%!important;
clear: both;
margin: 0 auto;
}
/* wrapper */
#wrapper {
width: 75%!important;
display: table-cell;
background: #ffffff url('images/blue/bg/bg_b_l.gif') top left repeat-y;
}