This is great advice. It worked perfectly for me.
I discovered that the cloned theme does not need to be active. It can be previewed when it is inactive, so there is no need to worry about users discovering it.
I read that with amusement. That was my thinking when I started exploring themes. I wanted to experiment and loaded a few different themes. The forum members found them very quickly.
To be doubly sure that your users do not get to see what you’re experimenting with, leave the “Theme can be selected by users” option unchecked. Users will be unable to choose the theme even if it is active.