In drupal 6 you could edit the user and it would give you the option to set the theme they use. This option does not appear to be available in Drupal 7. How else can one go about setting a theme on a per user basis?
This functionality has indeed been removed from core in Drupal 7, see the discussion here.
It seems the functionality has been superseeded by the ThemeKey module. Among other things it will allow you to specifiy themes for certain users/roles.
Hope that helps