Custom code zone

For one-off tweaks you do not need a developer or any code files: every section has Custom CSS and Custom Liquid boxes under Advanced customization in the theme editor. Those are settings, so updates preserve them.

For developers: the snippet zone

Three files belong to you and are never overwritten by a Vellum update:

snippets/custom-head.liquid renders at the end of <head> on every page
snippets/custom-body-start.liquid renders right after <body>
snippets/custom-body-end.liquid renders just before </body>

Walkthrough: an addition that survives updates

1. In the code editor, open snippets/custom-head.liquid and add your tag, for example a site-verification meta tag.
2. Save and confirm it renders on the storefront.
3. When a Vellum update arrives, follow the release's upgrade guide: the update process never touches snippets/custom-*.liquid, and our update-simulation test verifies that on every release before it ships.
4. After updating, your addition is still there. Nothing to re-paste.

The full engineering rules live in Updates that keep your work.