You can place PHP snippets at the bottom of your child theme functions.php file (delete "?>" if you have it there). CSS, on the other hand, goes in your child theme style.css file. Make sure you know what you are doing when editing such files - if you need more guidance, please take a look at my free video tutorial "Where to Place WooCommerce Customization?"
Does this snippet (still) work?
Please let me know in the comments if everything worked as expected. I would be happy to revise the snippet if you report otherwise (please provide screenshots). I have tested this code with Storefront theme, the WooCommerce version listed above and a WordPress-friendly hosting on PHP 7.3.
Check out these free video tutorials. You can learn how to customize WooCommerce without unnecessary plugins, how to properly configure the WooCommerce plugin settings and even how to master WooCommerce troubleshooting in case of a bug!
Business Bloomer Founder
Author, WooCommerce expert and WordCamp speaker, Rodolfo has worked as an independent WooCommerce freelancer since 2011. His goal is to help entrepreneurs and developers overcome their WooCommerce nightmares. Rodolfo loves travelling, chasing tennis & soccer balls and, of course, wood fired oven pizza.