When you create a presentation, PowerPoint automatically applies a theme: a coordinated set of colors, fonts, and effects. However, you can easily change the existing theme in a matter of seconds.
Click the Design tab.
Click one of the themes in the Themes group.
If none of the themes in the Themes group suits your needs, click the down arrow on the right side of the Themes box to display a gallery of additional themes.
Select the theme you want to use.
Select a variant to apply to your theme.
PowerPoint applies this new theme and its color variant to your presentation.
Tip
Preview a Theme Pause your mouse over each theme to preview it in your presentation.