A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Start with the new presentation open. Go to the Design tab. Click the down arrow on the right side of the Themes group to open the Themes gallery. Click Browse for Themes, which you'll find at the bottom of the Themes gallery. Browse to and choose the older presentation, the one whose theme you want to apply to the new presentation. Click OK.
Done.
See, every presentation contains the theme/template that it was built on, and PPT knows how to treat a regular PPTX as though it were a theme file.