Richer St-Amand is right. But the problem is that there are two rules inside a media-query, starting on line 3462 of the themes style.css:
/* Line 3492 */
.full-width .site-content .wp-caption.alignleft {
margin-left: -168px;
}
/* Line 3504 */
.full-width .site-content .wp-caption.alignright {
margin-right: -168px;
}
This rules can easily be overwritten with something like this:
.full-width .site-content .wp-caption.alignleft,
.full-width .site-content .wp-caption.alignright {
margin: 0;
}