Cornerstone was built with SASS, but you can still develop with normal CSS.

Instead, add this to your functions.php file:

<?php
function link_parent_theme_style() {
wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
wp_enqueue_style( 'child-style', get_stylesheet_uri() );
}
add_action('wp_enqueue_scripts', 'link_parent_theme_style');
?>

Typically, a child theme will @import the css of the parent theme. Bootstrap 3, the framework which Cornerstone is based on, offers very limited support for Internet Explorer 8. By default, a Bootstrap 3 site in IE8 will render the mobile view. One column of content, and the collapse mobile navigation.

Being that IE8 accounts for a sizable (~7%) of traffic on Aurora, Cornerstone makes use of respond.js, which uses javascript to mimic media queries for IE8. However, respond.js will not work on anything loaded via @import. So this alternate method is required.