Remembering that Drupal is arranged in "Regions", and that Zen tries to make things easy:

layout.css

sidebar-left-inner

{ margin: 0 20px 0 0; padding: 0; background-color:rgb(255,0,0); / added this line as a test / }

sidebar-right-inner

{ margin: 0 0 0 20px; padding: 0; background-color: #CDE8F3; / 2010-05-13 johnpfeiffer / }

background-image:url('/sites/default/files/images/bg-body-left.png')