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.