Wow! I just updated my personal site and the process was very seamless. I deleted my original Default theme, and I now only see the Horizon and Foundation themes in the list. Love it! 
Because I personally sometimes still prefer Foundation, I’ve set them both to be user selectable, and they now appear in the theme switcher in the bottom left corner.
I also set all the new horizon color palettes to be selectable by users. They were not selectable already. Now they all also appear in the color switcher in the bottom left corner, on Horizon. Has anyone figured out yet to have the color switcher for Foundation or any other theme besides Horizon?
This change is definitely worth calling out. While the Foundation theme cannot be edited you can effectively create your own theme with the same starting point and then edit it to you heart’s content.