[Kreativmind]

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Help Sizes and Dimensions
Main Table Width (pixels or percentage) (This is the width of the table that contains all your vBulletin content)

and in Additional CSS Definitions
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#side_borders_inner {
border-left: 1px solid #626262;
border-right: 1px solid #626262;
width: 790px;
margin: 0px auto;
}

#side_borders_outer {
border-left: 4px solid #EBEBEB;
border-right: 4px solid #EBEBEB;
width: 792px;
margin: 0px auto;
}