Add support for multiple themes (#4959)

* Add support for selecting a theme

* Fix codeclimate issues

* Look up site default style if current user is not available due to e.g. not being logged in

* Remove outdated comment in common.js

* Address requested changes in themes PR

* Fix codeclimate issues

* Explicitly check current_account in application controller and only check theme availability if non-nil

* codeclimate

* explicit precedence with &&

* Fix code style in application_controller according to @nightpool's suggestion, use default style in embedded.html.haml

* codeclimate: indentation + return

app/lib/themes.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'singleton'
+require 'yaml'
+
+class Themes
+  include Singleton
+
+  def initialize
+    @conf = YAML.load_file(Rails.root.join('config', 'themes.yml'))
+  end
+
+  def names
+    @conf.keys
+  end
+end