Choose File > New (from the File menu at the top of your screen), then double-click the theme you want to use as the basis for a new theme.
Custom themes appear in the My Themes category in the theme chooser. If you want to use a custom theme that doesn’t appear there, you must first install the custom theme (see the instructions above).
Make your changes, choose File > Save Theme, then choose an option to save the presentation as a new theme.
The edited theme is saved as a new version. You can rename the new version and delete the first version if you don’t want to keep it.
If you’re signed in to the same Apple Account on all your devices and Use the Finder to transfer Keynote presentations