Update theme.json example in Theme Handbook to use version 3 and correct data structure

[tomdevisser]

Issue Description

The Theme Handbook page on theme.json currently shows an outdated and partially incorrect example structure. The sample uses version 2, while the current recommended version is 3. Additionally, both customTemplates and templateParts are shown as objects, but they should be arrays.

URL of the Page with the Issue

​https://developer.wordpress.org/themes/core-concepts/global-settings-and-styles/#theme-json-structure

Why is this a problem?

This outdated and incorrect example may mislead theme developers, especially those new to working with theme.json. Using the wrong version or incorrect data structures can result in unexpected behaviour or errors when registering templates or parts, leading to frustration and unnecessary debugging.

Suggested Fix

Update the example to reflect the current standard format, using version 3, and ensure that both customTemplates and templateParts are shown as arrays. For example:

{
  "$schema": "https://schemas.wp.org/trunk/theme.json",
  "version": 3,
  "settings": {},
  "styles": {},
  "customTemplates": [],
  "templateParts": [],
  "patterns": []
}

[tomdevisser]

Just a quick follow-up — in addition to the example needing an update, there are other outdated references on the page. For example, right below the theme.json sample, the description for version still states:

The latest version is 2 and can always be found in the theme.json Living Reference…

This should be updated to reflect that version 3 is now the latest. There may be other similar mentions elsewhere in the Theme Handbook worth reviewing for consistency.