Add class to page based on topic category

app/assets/javascripts/discourse/models/topic.js

@@ -52,6 +52,10 @@ Discourse.Topic = Discourse.Model.extend({
     return null;
   }.property('category_id', 'categoryName'),
 
+  categoryClass: function() {
+    return 'category-' + Discourse.Category.slugFor(this.get('category'));
+  }.property('category'),
+
   shareUrl: function(){
     var user = Discourse.User.current();
     return this.get('url') + (user ? '?u=' + user.get('username_lower') : '');

app/assets/javascripts/discourse/views/topic_view.js

@@ -16,7 +16,8 @@ Discourse.TopicView = Discourse.View.extend(Discourse.Scrolling, {
   classNameBindings: ['controller.multiSelect:multi-select',
                       'topic.archetype',
                       'topic.category.read_restricted:read_restricted',
-                      'topic.deleted:deleted-topic'],
+                      'topic.deleted:deleted-topic',
+                      'topic.categoryClass'],
   menuVisible: true,
   SHORT_POST: 1200,