4
You don't have to do anything...altho the theme has 3 columns, the right column is not visible if no blocks are set to appear there.
Classes of center elements change to suit your column selection so if you have blocks only on left and center then center is bit bigger to fill the space and of right column. If you have blocks on both left and right column then the center will be smaller to fit in between
style.css if right column: center top/center bottom blocks width: 550px
style.css if no right column: center top/center bottom blocks width: 730px
same for the center-l, center-r, bottom-l, bottom-r they increase/decrease width to fit the theme.
So if you modified the theme files...you really didn't have to :)