In one of your skin template files, probably main.php, find these statements:
<div class="row {$SECTION}_wrapper">
<div class="small-12 large-9 columns" id="main_content">
<div>
<div data-alert="" class="alert-box alert">
<ul class="nomarg no-bullet">
<li>JavaScript is required by this website. Please enable this in your browser settings.</li>
Change the large-9 to large -12. The various CSS sizing classes instructs the browser how much of the available container to give to this row section. There are 12 columns per row (within the parent container). So, when the browser viewport is 'large', this section of 9 columns will take 75% of the row. A section of 12 columns will take 100% of the row.