This is an accepted solution.
For debut, the theme editor does not have variants for the nav buttons. You will have to edit the css in the "theme.css" file. These edits will depend on your preferences and you will need to be able to describe specifically the size, text, padding, border-radius... etc. for both mobile and desktop (you're not going to have a selection of buttons to choose from).
If you or no one on your team has a strong grasp of responsive css design, you should probably find another theme that you do not want to tweak as much.