Do any of the following:
Change the default theme
You can change the default theme to a theme that is included with PowerPoint or a theme that you create yourself.
If you haven't already, create and apply the theme that you want to set as the default.
For more information about creating a theme, see Customize and save a theme in PowerPoint for Mac.
On the Themes tab, under Themes, click the arrow next to Save Theme, and then click Set Current Theme as Default.
The next time that you open a new presentation, it will open with the theme that you set as the default.
Revert the default theme to the Office theme
On the Themes tab, under Themes, click the arrow next to Save Theme, and then click Reset Default to White Theme.