ThemeNegotiator not working for admin pages
The admin theme negotiator is tagged with a priority of -40:

user.services.yml

  theme.negotiator.admin_theme:
    class: Drupal\user\Theme\AdminNegotiator
    arguments: ['@current_user', '@config.factory', '@entity_type.manager', '@router.admin_context']
    tags:
      - { name: theme_negotiator, priority: -40 }


So you have to use at least -39, since the first negotiator returning a theme name wins and on an admin page this core service will return the admin theme.