Hm right.. But then adding a custom class to the body won’t do you any good either unless you use the add_editor_style.. but maybe you’ve thought of that already 🙂
I noticed the body element has an data attribute of the field key:
acf-editor-55cbc6fcb7ab7
So maybe you can target that
body[data-id="acf-editor-55cbc6fcb7ab7"]{
}