I've been playing with a distribution that includes Zen and a Zen sub-theme. I wrote some custom tpl and css files and then in a moment of sheer stupidity upgraded the distribution. Of course my custom files were wiped out in the upgrade and of course I had not archived them in Git. No real harm done because it was a sandbox-type project but it begs the question which I hereby pose to you: is it possible to create a sub-theme of a sub-theme? Is doing so a good practice or not? How does one, and should one, upgrade a distribution without overwriting any modifications?
TIA for all guidance rendered.