Add a plugin hook to specify a class on the body of the document

app/helpers/application_helper.rb

@@ -8,12 +8,16 @@ require_dependency 'mobile_detection'
 require_dependency 'category_badge'
 require_dependency 'global_path'
 require_dependency 'emoji'
+require_dependency 'multisite_class_var'
 
 module ApplicationHelper
   include CurrentUser
   include CanonicalURL::Helpers
   include ConfigurableUrls
   include GlobalPath
+  include MultisiteClassVar
+
+  multisite_class_var(:extra_body_classes) { Set.new }
 
   def google_universal_analytics_json(ua_domain_name = nil)
     result = {}
@@ -75,7 +79,7 @@ module ApplicationHelper
   end
 
   def body_classes
-    result = []
+    result = ApplicationHelper.extra_body_classes.to_a
 
     if @category && @category.url.present?
       result << "category-#{@category.url.sub(/^\/c\//, '').gsub(/\//, '-')}"

lib/plugin/instance.rb

@@ -103,6 +103,12 @@ class Plugin::Instance
     end
   end
 
+  def add_body_class(class_name)
+    reloadable_patch do |plugin|
+      ::ApplicationHelper.extra_body_classes << class_name if plugin.enabled?
+    end
+  end
+
   # Extend a class but check that the plugin is enabled
   # for class methods use `add_class_method`
   def add_to_class(class_name, attr, &block)