To delete a theme from your account, hover over a non-active theme from the menu selection and then click on the 'Delete' option.
When deleting a theme that is being used on forms, you will see a warning pop up that indicates you are using it on several forms and that those forms will be changed to the default theme for your account. You can then delete the theme and change those forms or cancel and go back without affecting any of your forms.
Once you have deleted the theme, any forms that used it will be immediately updated.