For showing “New” badge also in single product detail, you should add into function.php this

add_action( 'woocommerce_before_single_product_summary', array( 'WC_nb', 'woocommerce_show_product_loop_new_badge' ), 30 );

I recommend do this change by Child theme
And into Theme Options -> Custom Code -> Custom Css this:

.single-product #content span.wc-new-badge {
    left: 1px;
    position: absolute;
    top: 60px;
    z-index: 900;
}