Theme.json made its debut with the release of WordPress 5.8, coinciding with the introduction of Full Site Editing (FSE) and block-based themes. At its core, theme.json empowers you to configure the Block Editor more comprehensively. You can assign default styles to all or specific blocks, essentially defining how each block should look and behave. This extends the capabilities of the Block Editor API, enabling you to control the editor programmatically and integrate a block style system.
Theme.json empowers users to make site-wide stylistic changes, including controlling block features, color palettes, font sizes, and templates. It provides fine-grained control over website styling. To leverage theme.json, users need to use the Block Editor, have a basic understanding of CSS, and have access to the theme.json file in their theme’s root directory. However, not all themes support theme.json, so switching themes may be necessary.
Furthermore, theme.json was introduced alongside the Site Editor, marking a significant step toward managing styles for future WordPress releases. It provides you with greater control over your theme and site, allowing you to make changes to elements like menus, headers, and footers.