Apply Multiple Themes To Presentation. And a fun out-of-the-box Google Slide theme can help you deliver your lecture or lesson in a memorable and interesting way truly. The Design ribbon appears complete with a Theme gallery.

Click the Design tab. This is part 3 of the series of tutorials PowerPoint 2016. The theme that you set in the steps below will be applied to all of the slides in the current presentation.
It wont affect new presentations and it wont modify any existing themes on other presentations.
To add a second theme add another slide master with a different theme. Click the More icon at the bottom-right corner of the Themes section. When you apply a theme you apply attractive combinations of colors fonts and special effects like shadows reflections glows etc. By default the theme you select applies to all slides in your presentation.