How can I add CSS to a PHP script?

This is the script on which I'd like to add some CSS :

// Download URLs
        if ( $show_download_links && $_product->exists() && $_product->is_downloadable() ) {
            $download_files = $order->get_item_downloads( $item );
            $i              = 0;
            foreach ( $download_files as $download_id => $file ) {
                $i++;
                if ( count( $download_files ) > 1 ) {
                    $prefix = sprintf( __( 'Download %d', 'woocommerce' ), $i );
                } elseif ( $i == 1 ) {
                    $prefix = __( 'Download', 'woocommerce' );
                }
                echo "n" . $prefix . '(' . esc_html( $file['name'] ) . '): ' . esc_url( $file['download_url'] );
            }
        }
        // allow other plugins to add additional product information here
        do_action( 'woocommerce_order_item_meta_end', $item_id, $item, $order );
    }

It is part of a Woocommerce email (completed order). I'd like to add some CSS there, how can I do this ?