Bullet Train uses Devise for authentication and we've done the work of making the related views look pretty and well-integrated with the look-and-feel of the application template.
Bullet Train registers its own slightly customized registration and session controllers for Devise. If you want to customize them further, you can simply eject those controllers from the framework and override them locally, like so:
bin/resolve RegistrationsController --eject --open bin/resolve SessionsController --eject --open
You can customize Devise views using the same workflow you would use to customize any other Bullet Train views.