Hello, is it possible to duplicate the keystone theme? I modified the json and got the admin panel to read it as a theme, however when trying to apply the theme in generates a fatal error message.
I had the same issue as you, and while I don't have the solution, what I ended up doing instead was just duplicating one of the styles in the keystone theme instead of the whole theme, which was all I needed. Each style have a set of support files in the assets and design subfolders, as well as being listed in the main addon.json file.
Don't know if that helps you at all, but for me, it was the easiest option for creating my own style based on keystone.
I ended up using cssedit as a temporary solution to my issues, but I am going to try this now. Thanks for your time and the advice. I will update once I give it a shot!