Change default page (canvas) size with custom theme JSON not working

I am trying to change the default page size through a custom JSON theme. I have added the part below to the JSON theme file.

    "page": {
      "*": {
        "pageSize": [{
          "pageSizeHeight": 800,
          "pageSizeWidth": 1400,
          "pageSizeTypes": "Custom"
        }]
       }
    }

When I import the theme json, I get the message: File succesfully added. Your custom report theme is ready to use.

Unfortunately, the page size is still the default 16:9. Anyone knows how to fix this?