Apply a theme to your presentation
Open your presentation.
On the Design tab, you'll find design themes in the Themes group.
To preview how the current slide would look with a particular theme applied, rest your pointer over the thumbnail image of that theme.
To view more themes, on the Design tab, in the Themes group, click More More button at the right end of the gallery.
To select and apply a theme, click the thumbnail image of that theme.
Unless you specify otherwise, PowerPoint applies the theme you select to the entire presentation.
To apply a theme to only one or a few slides, select the slide or slides, right-click the theme you want, and select Apply to Selected Slides.