Set @body_classes to admin layout (#9081)

app/controllers/admin/base_controller.rb

@@ -5,8 +5,15 @@ module Admin
     include Authorization
     include AccountableConcern
 
-    before_action :require_staff!
-
     layout 'admin'
+
+    before_action :require_staff!
+    before_action :set_body_classes
+
+    private
+
+    def set_body_classes
+      @body_classes = 'admin'
+    end
   end
 end