How do I add CSS body classes for logged-in and logged-out users?
Ok I think I figured this out as in Drupal 8 it uses TWIG to print the body classes for content types or users we need.

In the html.html.twig  

{%
  set body_classes = [
    logged_in ? 'user-logged-in',
    not root_path ? 'path-frontpage' : 'path-' ~ root_path|clean_class,
    node_type ? 'page-node-type-' ~ node_type|clean_class,
    db_offline ? 'db-offline',
    user.hasPermission('administer')  ? 'isAdmin',
    user.getRoles()|join(' role_')
  ]
%}


Then make sure this line is added to the body classes

   &lt;body{{ attributes.addClass(body_classes) }}&gt;


So if the user is logged out we can use the role class to identify them. 

   .anonymous { }


Or we can use the :not Css selector 
:not

   :not.user-logged-in {}