Apply a theme to your presentation
Applying a theme is a fast way to give your slides an attractive, professional look. Apply a theme to all slides or some slides.
Select Design. Hover over a Theme to preview how it would make your slide look.
Select More 
to see more themes.
-
Click to select the theme you want to apply.
By default, the theme you select applies to all slides in your 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.
If you want to change the color scheme, fonts, effects or the background style, in the Variants group, select More 
.
Create and save a custom theme
Create a custom theme by modifying an existing one.
Select your first slide. On the Design tab, select the down arrow in the Variants group.
Select Colors, Fonts, Effects, or Background Styles to customize the theme.
Select the down arrow in the Themes group, and select Save Current Theme.
Source: Microsoft