How can I implement a bottom-up website fade on my ecommerce site?

[BIGROMBUSBILL]

I found the perfect example and being a shopify noob have trouble implementing it here it is: https://codepen.io/SitePoint/pen/warKXE the website is "happypawco.com"

We would like to implement this as the page is being scrolled, stopping at the footer.

Thanks for any support.