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/assets/stylesheets/variables.scss

-$color1: #282c37; // darkest
-$color2: #d9e1e8; // lightest
-$color3: #9baec8; // lighter
-$color4: #2b90d9; // vibrant
-$color5: #ffffff; // white
-$color6: #df405a; // error red
-$color7: #79bd9a; // succ green
-$color8: #000000; // black
+$color1: #282c37 !default; // darkest
+$color2: #d9e1e8 !default; // lightest
+$color3: #9baec8 !default; // lighter
+$color4: #2b90d9 !default; // vibrant
+$color5: #ffffff !default; // white
+$color6: #df405a !default; // error red
+$color7: #79bd9a !default; // succ green
+$color8: #000000 !default; // black

app/helpers/style_helper.rb

+# 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

app/views/layouts/application.html.haml

@@ -17,7 +17,7 @@
         = ' - '
       = site_title
 
-    = stylesheet_link_tag 'application', media: 'all'
+    = stylesheet_link_tag stylesheet_for_layout, media: 'all'
     = csrf_meta_tags
 
     = yield :header_tags

config/initializers/assets.rb

@@ -8,5 +8,6 @@ Rails.application.config.assets.version = '1.0'
 
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.
-Rails.application.config.assets.precompile += %w(application_public.js)
+Rails.application.config.assets.precompile += %w(application_public.js custom.css)
+
 Rails.application.config.assets.initialize_on_precompile = true