The CKEditor configuration has a property 'contentsCss' that can contain one or more CSS files to include in a CKEditor field. The default CKEditor configuration uses this property to include the file 'ckeditor/hippocontents.css' with the default styling. Users can add their own styles by overriding this property and adding their own style sheet .
This contentsCss property can be a string with one stylesheet, or an array with multiple stylesheets. Because the default value of contentsCss (in CKEditorNodePlugin.java and CKEditorPropertyPlugin.java) is a string, users always have to override it in ckeditor.config.overlayed.json. That forces them to 'repeat' the hippocontents.css style definition, otherwise the default styling is gone.
It would be nicer if the default contentsCss property value was an array with one element. Users can then add their own additional stylesheet to ckeditor.config.appended.json without having to repeat the default stylesheet.