Hello,
Code will work from both areas, also, custom code can be added in style.css file of the child theme, but we would recommend adding it via Theme Custom CSS because we are responsible for the work of this area. In case your correct code won’t work at all from the Additional CSS area, you will need to contact WordPress support because it is independent of our theme.
If you have any additional questions or concerns, please do not hesitate to reach out to us. Our support team is always available to assist you.
Kind Regards,
8theme team