You can apply as many designs into a PowerPoint presentation as you'd like. Here are the steps to apply more themes to your presentation.
1. Open your PowerPoint presentation. On the View tab, under the Master Views group, click Slide Master.
2. On the left slide list, scroll down and click underneath the list of slide thumbnails to get a blinking line to add a new master set.
3. On the Slide Master tab, under the Edit Theme group, click Themes and choose Browse for Themes...
4. browse your computer to find the template you want to add to your presentation, select the template and click Apply.
5. the template will be added as a new slide master set. On the Slide Master Tab, click Close Master View to return to editing your presentation slides.
6. Click on the slide you want to apply your new template theme to by clicking on that slide in the left slide list.
7. On the Home tab, under the Slides group, click on Layout and you will see all of the new layouts added with the new template. Choose the layout you want to apply to the slide will update the background, placeholders and theme colors based on that template design.