Adding customized themes to your library is a great way to jump-start into your presentation.
To save a theme to your library:
Step 1: Click open the presentation with the theme-template you want to save.
Step 2: Click on Edit Theme on the left
Step 3: Click on the Save To Theme Library button at the bottom left corner
Step 4: Name your theme, and then click OK to save
Step 5: Go back to your home page and click on Create New Presentation. A list of your themes will appear with your new saved theme: