If you want to set the default aspect ratio in PowerPoint 2013 to the standard 4:3 every time you open PowerPoint and create a new presentation, then you can do that simply. Just need to change the default size to 4:3, then save the current theme, and then set the new saved theme as Default. So everytime you open PowerPoint next time. (See How to Change Slide Size in PowerPoint 2013 to 4:3 Aspect Ratio)

And now you can set as Default Theme. Now, every time you create a new presentation then the new theme will be created.

You can learn more in the article How to change the slide size to standard or widescreen in Powerpoint 2013

Via: Slidehunter