IMO the A mode (which was the only mode when we first went to Discourse) is the best of both worlds. At least on a real computer, not a phone.
You can see everything you enter and also see the actual result of your clicks on the menu bar buttons, be that bold or insert poll or … . While side by side over on the right, you can see the result as it will look once posted.
Themes are a whole different kettle of fish; best not to intrude them into this discussion.