How to set a default template for each new presentation in PowerPoint

To begin, open your template in PowerPoint (I promise, this is the last time you will have to do this). Select the Design tab in your toolbar and expand the list of themes:

Click on Save the active themeand give it the name you want, then click on Save. Expand the list of themes again and you will find your newly created theme in the Custom. You just have to right click on it and select the option Set as default theme.

From now on, your theme will appear every time you launch PowerPoint :

However, you can always start from an empty presentation using the New Presentation.