Themes in PowerPoint 2013 make it easy for you to apply a harmonious, coordinated look to the presentation you’re creating. Each theme includes a number of design choices that control how the colors, fonts, and effects look in your presentation. You can get more specific with your design choices by choosing one of the new PowerPoint 2013 variants, which display the same theme in a variety of colors from which you can select.
Open the presentation that you want to use.
Click the Design tab.
In the Themes group, click the arrow to display additional theme choices.
A gallery of themes appears.
Click the theme that you want to apply to the current presentation.