#sidebar-first + #content-wrapper do not fit in #page

http://d-theme.com/acquia_slate shows a width problem. #page is 960px wide. #content-wrapper is 960px wide. There is no room left for #sidebar-first. I experimented and found #sidebar-first fits when #content-wrapper is reduced to 820px. The installation is a default Drupal 6.12 with the theme configuration left at the defaults.