Use mime type symbol to set content_type for custom css response (#37845)

app/controllers/custom_css_controller.rb

@@ -3,7 +3,7 @@
 class CustomCssController < ActionController::Base # rubocop:disable Rails/ApplicationController
   def show
     expires_in 1.month, public: true
-    render content_type: 'text/css'
+    render content_type: :css
   end
 
   private