Add CSS to admin area

function customAdmin() {
    $url = get_settings('siteurl');
    $url = $url . '/wp-admin/css/woo.css.';
    echo '<!-- custom admin css -->
          <link rel="stylesheet" type="text/css" href="' . $url . '" />
          <!-- /end custom adming css -->';
}
add_action('admin_head', 'customAdmin');