This document will show you how to save a PowerPoint theme to your computer.
Open up the PowerPoint file you want to save as a theme.
Go to the "Design" tab in the ribbon.
In the ribbon, under "Themes", click the "More" drop-down arrow.
Click "Save Current Theme...".
Give the theme a name and click "Save".
Now, under the "Design" tab, you will have that theme available anytime you open PowerPoint on your computer. This also allows you to apply the theme to an already created presentation without having to recreate it.
Note: This only saves it for your username on that specific computer. You will need to repeat these steps if you want it on another computer or for another user.