public function addMagentoCss($observer)
    {
        if (!Mage::helper('australia/address')->isAddressValidationEnabled()) {
            return;
        }

        /** @var Mage_Core_Model_Layout $layout */
        $layout = Mage::getSingleton('core/layout');

        /** @var Mage_Page_Block_Html_Head $head */
        $head = $layout->getBlock('head');

        $skinBaseDir = Mage::getDesign()->getSkinBaseDir(array('_package' => 'base', '_theme' => 'default'));
        $cssPath = 'prototype/windows/themes/magento.css';

        if (file_exists($skinBaseDir . DS . 'lib' . DS . $cssPath)) {
            $head->addCss('lib' . DS . $cssPath);
        } else {
            $head->addItem('js_css', $cssPath);
        }
    }