you could probably define left/right borders in the css. Its hard to guess how that would work for your theme unless you provide a link though.
As the content and sidebar divs are not full height, the best way to achieve this would probably be to add to the #wrapper element a 1px tall repeating background image to define the line.
Try adding this to the Custom CSS box und <b>Appearance > Brunelleschi Options</b>:
#main {
border-right: 1px solid #CCC;
border-left: 1px solid #CCC;
}
