Setting _admin_route for all forms steps route is too agressive.

Problem/Motivation

If the "Use the administration theme when editing or creating content" appearance setting is enabled, the RouteSubscriber for this module will automatically set all forms_steps routes to be an _admin_route regardless of whether the route is for a node entity.

Normally, that setting is only applied to node operation routes (see \Drupal\node\EventSubscriber\NodeAdminRouteSubscriber). The way that \Drupal\forms_steps\EventSubscriber\RouteSubscriber::routes is written, if that setting is enabled it applied to all entity types, even custom ones.

This was implemented in #3104208: Using the admin theme for editing via 'Steps' → .

We had to write a custom RouteSubscriber to undo the setting for our custom entities. Not a big deal, but might be confusing to some folks.

Proposed resolution

One of these could be a solution:

1. Check if the forms_steps config is for a node entity type before using the node.settings.use_admin_theme setting value.
2. Add an additional option to forms_steps configs to allow a "Use the administration theme" option on a per forms_steps basis.