In DNN7, you could load the editor.css file if you wished to add your own styles to the editor. We used this feature to add a class for links that was linked to a speedbump warning, telling site visitors that they were leaving that site for one controlled by other parties. Clients clicked on the CssClass menu, and selected the class (we removed all the defaults, and left about 5-7 per client so they weren't overwhelmed).
On DNN5, clients had to note or remember the class names, and add them on the Advanced pane of the Add a Link window. It seems like DNN9 has rolled back to this DNN5 setup....CKEditor claims to allow you to set up your own Styles Drop-down, but when I edit anything in the Editor Config tab of the CKEditor Provider Settings and hit Save (and refresh), my Persona bar appears in black on the left, but without tools. On Inspect Element, it looks like the iframe of the Persona Bar guts just aren't being loaded at all.
Additionally, Publish may or may not work, and if it works (not sure), the window that says "Your page has been published!" does not go away, the Publish/Discard menu at the bottom does not go away, and the page does not reload with the updated content.
While some of this issue may be caching - because I can go to a private window in Firefox, or try Chrome, and the Persona Bar loads fully and I can once again edit - if I attempt to edit the Editor Config settings again, the problem starts again. Sometimes after clearing my cache, the problem persists. Sometimes I can "go in and edit again," but the Rich Text Editor does not appear in Advanced Editing. Since we're in the beginning stages of how we can start creating sites on DNN9 and still provide the features our clients need, this inability to test or edit is highly frustrating.
Chrome seems a little better - like it's not shutting off the Persona Bar, but then I have an unrelated issue to my Chrome misbehaving with fonts, so I avoided using that browser in the past. But even in Chrome, after I edit the Config, the rich text editor is not displaying.