Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / node / migrations / d7_node_settings.yml

id: d7_node_settings
label: Node configuration
migration_tags:
  - Drupal 7
  - Configuration
source:
  plugin: variable
  variables:
    - node_admin_theme
  source_module: node
process:
  use_admin_theme: node_admin_theme
destination:
  plugin: config
  config_name: node.settings