utilizing bootstrap gem pick your own sass library code

Gemfile

@@ -5,4 +5,7 @@ gemspec
 gem 'rails'
 gem 'i18n-tasks'
 
+# For development; when running as a library, you can pick your own sass compiler
+gem 'dartsass-sprockets'
+
 eval_gemfile './shared.gemfile'

README.md

@@ -20,6 +20,12 @@ Add [![Gem Version][gem-badge]][gem] to Gemfile:
 gem 'rails_email_preview', '~> 2.2.3'
 ```
 
+This gem requires a Sass engine, please ensure you have **one** of these gems in your Gemfile:
+- [`dartsass-sprockets`](https://github.com/tablecheck/dartsass-sprockets): Dart Sass engine, recommended but only works for Ruby 2.6+ and Rails 5+
+- [`dartsass-rails`](https://github.com/rails/dartsass-rails): Dart Sass engine, recommended for Rails projects that use Propshaft
+- [`cssbundling-rails`](https://github.com/rails/cssbundling-rails): External Sass engine
+- [`sassc-rails`](https://github.com/sass/sassc-rails): SassC engine, deprecated but compatible with Ruby 2.3+ and Rails 4
+
 Add an initializer and the routes:
 
 ```console

lib/rails_email_preview.rb

@@ -5,7 +5,25 @@
 require 'rails_email_preview/delivery_handler'
 require 'rails_email_preview/view_hooks'
 
-require 'dartsass-sprockets'
+# sass engine loader
+begin
+  require 'dartsass-sprockets'
+rescue LoadError
+  begin
+    require 'sassc-rails'
+  rescue LoadError
+    begin
+      require 'dartsass-rails'
+    rescue LoadError
+      begin
+        require 'cssbundling-rails'
+      rescue LoadError
+        raise LoadError.new("bootstrap-rubygem requires a Sass engine. Please add dartsass-sprockets, sassc-rails, dartsass-rails or cssbundling-rails to your dependencies.")
+      end
+    end
+  end
+end
+
 require 'request_store'
 require 'turbolinks'
 require 'pathname'

rails_email_preview.gemspec

@@ -18,7 +18,6 @@ Gem::Specification.new do |s|
   end
 
   s.add_dependency 'rails', '>= 4.2'
-  s.add_dependency 'dartsass-sprockets'
   s.add_dependency 'turbolinks'
   s.add_dependency 'request_store'