If you don’t like the default themes PowerPoint has available, you can create your own theme by saving any combination of theme colors, theme fonts, theme effects, and background styles.
Once you’ve modified a theme, it’s easy to save it for future use.
Click the Design tab on the ribbon.
Click the Themes gallery More button.
Select Save Current Theme.
Enter a name for the theme and click Save.
Now when you open the Themes gallery, your custom theme will be listed alongside the rest of the Office themes. Any custom themes you save are available in all Office programs.