PERF: missing caching on CSS and Site Customizations

app/controllers/site_customizations_controller.rb

@@ -2,6 +2,13 @@ class SiteCustomizationsController < ApplicationController
   skip_before_filter :preload_json, :check_xhr, :redirect_to_login_if_required
 
   def show
+    version = params["v"]
+
+    if version && version == request.headers['If-None-Match']
+      return render nothing: true, status: 304
+    end
+
+    response.headers["ETag"] = version if version
     expires_in 1.year, public: true
     render text: SiteCustomization.stylesheet_contents(params[:key], params[:target] == "mobile" ? :mobile : :desktop),
            content_type: "text/css"