How do you deselect a theme once you've applied it to your form?

Unfortunately you can no deselect a theme once it was already applied, but you can go back to the version of the form when the theme was not already applied.

You can do that by following the steps here: How to View Form Revision History

It will however remove all the other changes that you make to the form, such as changes in email, thank you page, conditions, form fields, etc, so if you are not exactly sure if you want that to happen, I suggest cloning your current form first to make sure that you have a reference or a backup form.