form_mode_manager-8.x-1.x-dev/modules/form_mode_theme_switcher/form_mode_manager_theme_switcher.services.yml

services:
  theme.negotiator.form_mode_theme_switcher:
    class: Drupal\form_mode_manager_theme_switcher\Theme\FormModeThemeNegotiator
    arguments: ['@config.factory', '@current_user']
    tags:
      - { name: theme_negotiator, priority: -30 }