I have registered a custom post type using
register_post_type('myposttype', ...). On published posts, there is a custom class that gets applied to the whole body of the page,
single-myposttype. I apply css rules using this body class to make the post appear differently from non-custom post types posts.
Now that the Gutenberg editor is such a key part of Wordpress, I would like to be able to add a custom body class to the editor when working on these custom post types, so that the same style rules that apply to the published post are applied during editing.