Compensate for scrollbar disappearing when media modal visible (#8100)

* Compensate for scrollbar disappearing when media modal visible

Make auth pages backgrounds lighter

* Fix typo

app/controllers/auth/registrations_controller.rb

@@ -8,6 +8,7 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   before_action :configure_sign_up_params, only: [:create]
   before_action :set_sessions, only: [:edit, :update]
   before_action :set_instance_presenter, only: [:new, :create, :update]
+  before_action :set_body_classes, only: [:new, :create]
 
   def destroy
     not_found
@@ -79,6 +80,10 @@ class Auth::RegistrationsController < Devise::RegistrationsController
     @instance_presenter = InstancePresenter.new
   end
 
+  def set_body_classes
+    @body_classes = 'lighter'
+  end
+
   def set_invite
     @invite = invite_code.present? ? Invite.find_by(code: invite_code) : nil
   end