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:

.full {
.main-page {
margin: 0 auto;

And that should get you rolling on a full width custom theme based on _s.