To delete a Theme, go to the Themes menu and select Delete. The delete dialogue box will appear. Read the "Confirm Delete" message and then click "Delete" or click "Leave" if you do not wish to delete or are unsure. If a Theme is used in a survey you will be given a warning message and you will have an option to assign another Theme.
When a project is launched, a copy of the entire survey, including the themes is made. Editing or deleting a Theme will not change the Theme in the launched project.
Multiple themes can be deleted by selecting the themes and then choosing the Selected/Delete menu option at the top.