How to install this theme in Shopify?
Picksy is a headless commerce application based on Shopify.
You won't be able to install this theme in Shopify as it will be a separate entity.
To know more about headless commerce please check this article
https://www.shopify.com/enterprise/headless-commerce
https://www.gatsbyjs.com/solutions/shopify/
TLDR; it's not possible.