Changing the default design for new presentations is a little different in the latest version of PowerPoint. To set a new default, do the following…
To change the default design to an existing theme in the theme gallery:
1. Go to the Design tab.
2. Right-click on any theme in the theme gallery.
3. Select Set as Default Theme.
To make your own template for new presentations:
1. Create a new template and design it as needed.
2. Save the template as blank.potx.
It will now be the default design used when opening up new blank presentations.