Themes use a templating language called Liquid 🤓
It provides control structures, variables, and other functionality for HTML, CSS, Javascript, and any other text based file.
Our documentation on theme code can be found on our GitHub Repository.
Liquid code is originally created by Shopify, and their documentation is a great starting point.
If you have any questions or need further support regarding Liquid, feel free to create an Issue on our GitHub Project.