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/about_controller.rb

@@ -26,7 +26,7 @@ class AboutController < ApplicationController
   end
 
   def set_body_classes
-    @body_classes = 'about-body'
+    @body_classes = 'with-modals'
   end
 
   def initial_state_params