I just wrote a blog post on how I was able to use the wrapper CSS Class field on the WYSIWYG to modify the body class within the rendered iframe:
http://hereswhatidid.com/2015/08/customize-acf-wysiwyg-input-styles/
I also have a fix in place that would add a new property to all WYSIWYG fields that lets you directly specific the classes rather than using the filter but I’m waiting on that to be reviewed.
Let me know what you think, thanks!
