How to Borrow a Theme from Another PowerPoint Presentation
On the Design tab, open the Themes gallery.
The Themes drop-down list opens.
Click Browse for Themes.
Choose Theme or Themed Document dialog box.
Locate and select a PowerPoint presentation with a theme you can appropriate for your presentation.
You may have to browse to find the presentation you want to use.
(Optional) Examine the themes.
Click the Views button in the upper-right corner of the dialog box and choose Preview.
Click the Apply button.
The theme used in the presentation you selected now inhabits your presentation.