? From the screen shot it looks like you've already done this with something like:
#region-main {
padding-left: 0;
padding-right: 0;
}
Unless you then change the value of the selectors:
.course-content ul.ctopics li.section.main .side,
.course-content ul.ctopics li.tcsection .side
by changing the string:
$string['topcollsidewidthlang'] = 'en_us-28px';
in /course/format/topcoll/lang/en_us/format_topcoll.php then do a 'Purge all caches' or use Moodle's own language string editor in the UI to change that string for my Collapsed Topics course format.