By 'copying it': do you mean TTW ? If so, try to upgrade to latest version of Plone 5.0.2 or 5.0.3. I saw this behaviour on one of the early (beta?) versions of Plone 5.
I happened to me once when testing Plone. But then, in another instance with the same buildout it works fine. Thus, I could not reproduce the issue again.
Yes, i can reproduce this Problem with my Plone 5.0.2 System. But you can go to the Resourceregistry Controlpanel and recompile the Thememapper Bundle, i enable the Development Mode also, and now the Theming Editor is running. Best Regards, Jan