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

# Admin UI - Theme switcher
form_mode_manager.theme_switcher_settings:
  title: 'Form Mode theme switcher'
  description: 'xxx'
  weight: -10
  parent: form_mode_manager.admin_settings
  route_name: form_mode_manager.theme_switcher_settings