Drupal 5 not allow the node-specific admin theme.

migration_templates/d5_node_settings.yml

-id: d5_node_settings
-label: Drupal 5 node configuration
-migration_tags:
-  - Drupal 5
-source:
-  plugin: variable
-  variables:
-    - node_admin_theme
-process:
-  use_admin_theme: node_admin_theme
-destination:
-  plugin: config
-  config_name: node.settings