clicking that button will show the HTML code of your answer. If the answer is empty, nothing will be shown. If you have entered some content, then you should see the HTML code of that content, something like this:
If you don't use headers, then you won't see h3/h4.
To customize the quiz screen using a remote CSS, I recommend you to use the webapp (in Chrome or Chromium) to inspect the elements you want to customize, and then write the CSS rules you need. As I said in another post, I suggest you to add "ion-content.mm-site_mod_quiz-player" to all your rules to apply them only in the quiz player.
If you want to customize the app using a remote theme I recommend you to take a look at the presentation I linked in another post in this thread.