Those themes are for the dummy data import. An action you do when you first install theme and have no pages/settings set up. After importing the desired demo, you do not need to access it again anymore. So “editing default theme” does not have really a meaning as you do not edit templates.
If you want the client to not have access to this area then it is better to install a role management plugin and assign what menu items to let him have access to, and which not (in this case forbidding to see/access the “Import/Export (Dummy Data)” menu item).
Or add a custom css in your custom css box, deleting the ones you don’t want to see.
