The customer has a wordpress site (now version 4.2.2) with a custom theme that was made somewhere on the side. The WP engine itself has not been changed, it is updated with built-in tools successfully. The internal service name of the theme seems to conflict with some standard WP theme, and the admin panel strongly suggests updating the theme. The test update in the sandbox all broke - in general, can not be updated.
How can I rename a custom theme so that WP understands that it was custom and did not try (did not offer) to update it?