With WordPress 5.8, it is now possible to configure the editor through a new mechanism: theme.json. This addition means that themes can provide more control over what editors should do when editing posts or pages on your website (change font size, color schemes etc.).
With the addition of theme.json, theme authors can also control these settings on a per-block basis, which wasn’t possible before. The top-level applies to all blocks that support the relevant setting, but a specific block’s settings could override it by being more specific.Read Article →