# get array of stylesheets
$css = drupal_add_css();
unset($css['all']['module']['modules/system/system.css']);
unset($css['all']['module']['modules/system/defaults.css']);
unset($css['print']['theme']['themes/mytheme/print.css']);

stylesheets-conditional[lte IE 7][all][] = _css/ie7.css

@import url("ie7.css");