Try adding this PHP snippet:
add_action('generate_before_header', function(){
echo '<div class="body-inner grid-container">';
},10);
add_action('wp_footer', function(){
echo '</div> <!-- body inner -->';
},9999);
This wraps both header, content (and footer) with a <div>
while honoring the max-width imposed to them.
You then replace this CSS:
.site-header, .container, .site-footer, .site-info {
box-shadow: 0 10px 10px #555;
}
With this CSS:
.body-inner.grid-container {
box-shadow: 0 10px 10px #555;
}