unique theme for unique user

Good day . i would like to be able to give a specific users unique looks that the other subscribers to my blog cannot access unless i activate it for them .. so lets say mr.x is a construction worker i would like to make a child theme for him and add it to his blog ,, but only to his blog .. it may not be available to other users to choose from in the theme selection area . is there a way i could do this .. ?