Get GhostCMS custom theme settings via Ghost Content API

Question

I have custom GhostCMS theme and inside its package.json:

...
        "custom": {
            "custom_setting_key": {
                "type": "text",
                "default": "Logo on cover"
            },
...

and I see it in the admin -> Site Wide. I can change it, but I can't get it via GhostCMS Content API. The other default settings are there, but not any from the custom property:

http ... site_url/ghost/api/v4/content/settings/?key=key here

Is there a standard way to get them via the API?