Custom css variables

{
    "version": 2,
    "settings": {
        "custom": {
            "line-height": {
                "body": 1.7,
                "heading": 1.3
            }
        }
    }
}

will be accessible in css --wp--custom--line-height--body

Note: the custom property can be add into a block, thus enabling overriding.

Sources :

• https://developer.wordpress.org/block-editor/how-to-guides/themes/theme-json/#css-custom-properties-presets-custom
• https://developer.wordpress.org/block-editor/how-to-guides/themes/theme-json/#custom