Use Sprockets outside Rack application. I'm working on a script project which is non Rack based. I want to send users an email notification when the script detects a state change. To use Sprockets outside a Rack application is not a common case, so there are not many articles talking about it online. You could count on this.

use-sprockets-ouside-rack-application.rake

desc 'Compile scss files using Sprockets

+ source file: #{root}/lib/assets/stylesheets/application.scss, 
  in which you could require the third party assets
+ output file: #{root}/build/emai.css
'
task :compile do
  require 'third_party_assets'

  root = '~/Workspace/project/'

  ROOT        = Pathname(root)
  LOGGER      = Logger.new(STDOUT)
  BUILD_DIR   = ROOT.join('build')
  BUILD_FILE  = 'email.css'
  SOURCE_DIR  = ROOT.join('lib/assets')
  SOURCE_FILE = 'application.scss'

  environment = Sprockets::Environment.new(ROOT) do |env|
    env.logger = LOGGER
  end
  environment.css_compressor = :scss

  environment.append_path(SOURCE_DIR.join('stylesheets').to_s)

  assets_path = Bundler.rubygems.find_name('third_party_assets').first.full_gem_path
  environment.append_path("#{assets_path}/app/assets/stylesheets")

  assets = environment.find_asset(SOURCE_FILE)
  assets.write_to(BUILD_DIR.join(BUILD_FILE))
end