Our documentation site currently lives at https://primer.style/presentations/. You'll be able to find the information listed in this README as well as detailed docs for presentation guidelines, presentation formats, and assets.
To install any of the Keynote themes, simply download the theme you'd like to use and double-click on the Keynote theme file (.kth). This will open Keynote with the dialog box that looks like this: 
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Click Add to Theme Chooser. The theme will now be available in the My Themes tab when you create a new presentation.
To install the theme from a pre-existing presentation, use the Keynote template file, simply download the file and double-click the Keynote file (.key) to open.