The Underscores theme, _s, from Automattic comes, by default, with a fixed width layout. In order to update to a full width layout, you’ll need to replace the div id=”page”.
Screenshots of Header.php and Footer.php are shown below.
Header.php, BEFORE updates:
Header.php, AFTER updates:
Footer.php, AFTER updates:
And, of course, add a few lines to your style sheet:
margin: 0 auto;
And that should get you rolling on a full width custom theme based on _s.