You can easily change the look and feel of your presentation by applying a different theme. Themes have associated color schemes, fonts, and effects to help give your PowerPoint presentation a unified, polished appearance.
On the Design tab, in the Themes group, click More to open the entire gallery of themes.
Tip: To preview how the current slide looks with a particular theme applied, rest your pointer on the thumbnail of that theme.
Select the theme you want to use.
In the Variants group, select a color variation that you like.