Cloned Seven/Bartik themes don't show up
I renamed all the files named bartik to xbartik


When you say all the files, I hope you also did bartik.schema in the /config/schema folder.


  I did a find a replace in the info and theme files to the
  corresponding name change of xbartik and xseven.


You need to do this for all files, including files like bartik.schema, bartik.libraries.yml bartik.breakpoints.yml.