When you're on the slide you want to change, choose "Slide Design..." from either the Format menu or the right-click contextual menu. Near the bottom of the box that pops up, there's a radio button to choose either all slides or only the current slide.
Check out this video tutorial, it shows you how to create a ppt with theme's and how to have multiple theme's in one ppt by adding sections 🙂 https://www.youtube.com/watch?v=RQ0webuhJFk