I have created a custom theme (copying antimatter to a new folder) and I have been able to use it by modifying the system.yaml file. However my theme doesn’t appear in the configuration back office or in the drop down for selecting a theme. If I save any changes to the configuration it automatically gets set back to antimatter.
thanks, I think it was a mixture of not all renames being lower case, once i made sure they were all the same it seemed to work okay and the custom theme was selectable in the admin area