Add your own sass helpers if you're using the sass-rails gems

base.css.scss

html {
  background: background_url_from_config() no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

sass_functions.rb

#with sass-rails you get some helpers like asset-url and asset-path
#you can add your own like this
module Sprockets
  module SassFunctions
    def background_url_from_config
      Sass::Script::String.new("url(#{ENV['CDN_BG_IMAGE']})")
    end
  end
end