theme.json v2 renamed the keys you just learned

Version 2 arrives with 5.9, six months after version 1, and moves several keys — a file written in 2021 needs migrating.

{
  "version": 2,
  "settings": {
    "appearanceTools": true,
    "layout": { "contentSize": "48rem", "wideSize": "72rem" },
    "spacing": { "blockGap": true }
  },
  "styles": {
    "elements": { "link": { "color": { "text": "var(--wp--preset--color--brand)" } } }
  },
  "templateParts": [ { "name": "header", "area": "header" } ]
}

The customTemplates and templateParts sections are new and are the parts that a classic theme adopting v2 does not need, which is worth knowing before reading a migration guide written for block themes. layout.contentSize replacing the theme support call is the change with the widest blast radius, because it now controls the width of every constrained group on the site.