Delete a theme and go back to NO theme

To delete themes, click on a theme and it will popup the theme details. There is a delete link at the lower right hand corner of the popup.

You must have at least one theme to run a blog. Usually the default is twentyfourteen. Is that what you meant by no theme?

https://ts.w.org/wp-content/themes/twentyfourteen/screenshot.png?ver=1.2

When you get to your last theme, you cannot delete it because WP needs at least one theme to be present.