by Davo Smith.
Have you tried extending the quiz styles.css with (this is off the top of my head and untested):
#page-mod-quiz-attempt .side-pre-only #page-content #region-main {
margin-right: 200px;
}
This is assuming you want it to be on the quiz attempt.php page, if it isn't, you need to check the id of the body tag on the page you are trying to edit (via 'inspect element' in chrome / firebug).
You will, of course, need to either purge the site cache via the admin menu or turn on 'theme designer' mode before any changes in CSS files take effect.