Answer:
when you add a new slide
Step-by-step explanation:
When creating a slide in a presentation you can add a different design template that might be different from the other templates used when creating previous slides.
You cannot add a new template to a presentation when you have already started the presentation that will be unethical practice and will end up confusing your audience and also adding a design template before the close of the program will be of no use and will not pass any information instead it will make your work to look disorganized at the end of the presentation.