Well, I thought conditionals work in css too. My bad. They only work in html.
You need this code in functions.php of your child theme:
add_action('wp_head', 'add_ie8_custom_css');
function add_ie8_custom_css {
echo '<!--[if IE 8]>
<link rel="stylesheet" type="text/css" href="'.get_stylesheet_directory_uri().'/ie8-hacks.css" />
<![endif]-->'
}Should work now. I'm sorry I can't test this myself, I don't have IE lower than 10 here and this type of conditionals do not work in IE10 and above.