You're abusing a browser action button. If you want to have an option page, just define it in your manifest as an option page. What you have now is an option page hidden behind a browser action button... :S
Try updating the css live. Not sure how your code works, but it makes it far easier to understand what various features do and if you're properly saving your settings there is listener you can listen on for changes.
It might make sense to either automatically update the length of the input field or scroll the preview field the same relative amount. Option 2 is a lot wiser probably... though I like option 1 better xD .
Stylish design option seems to be always turned on.
Honestly, issue #1 is for me bad enough that I am tempetd to just uninstall the extension, but I quite like the work you did on the editor as I have been considering doing the same thing myself. Oh well, guess I will just try hiding the browser action button behind the arrow for the time being.