Check for a custom css file to help customization of instances (#1368)

* User can create a custom.scss to customize their instance without modifying gitted files.

* Add documentation for customization.

* Forgot the helper file

* Fix Style to pass codeclimate

* Requests from maintainer.

app/helpers/style_helper.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module StyleHelper
+  def stylesheet_for_layout
+    if asset_exist? 'custom.css'
+      'custom'
+    else
+      'application'
+    end
+  end
+
+  def asset_exist?(path)
+    if Rails.configuration.assets.compile
+      Rails.application.precompiled_assets.include? path
+    else
+      Rails.application.assets_manifest.assets[path].present?
+    end
+  end
+end