Got it. Basically we're moving `['all']` to `['(min-width: 720px)']` in the array structure. When it's loaded back in *all* of the stylesheets are effectively now wrapped into a media query.


    function hook_preprocess_page(&$vars) {
      // get style sheet array
      $css = $vars['css'];

      // copy over the [all] to a media query key
      $css['(min-width: 720px)'] = $css['all'];

      // Remove the all
      unset($css['all']);
    
    
      $vars['styles'] = drupal_get_css($css);
     }

This leaves us the ability to define other style sheets in `theme.info` for other breakpoints.  

    stylesheets['(max-width: 480px)'][] = phone.css