Theme 25 Responsive Grid Column Visualization
<link rel="stylesheet" href="http://www.themes4apex.com/blog/grid_background.css" type="text/css">
in your template, or just as URL in the template or page’s CSS URLs property.
(I uploaded an enhanced version of the CSS after the suggestion from Patrick Wolf, to only show the grid columns when the developer clicks the "Show Grid" option in the Developer Toolbar. Thanks, Patrick. I should have thought of this myself.)