Convert all your css files to sass

css2sass.rb

require 'rubygems'
require 'sass/css'

Dir["/PATH/TO/YOUR/PROJECT/app/assets/stylesheets/**/*.css"].each do |file|

  puts "==> Starting #{file}..."

  # file properties
  basename = File.basename(file, ".css")
  dir = File.dirname(file)

  # convert to new file
  old_data = IO.read(file)
  new_data = Sass::CSS.new(old_data).render(:sass)

  # save to file.sass
  File.open("#{dir}/#{basename}.sass", 'w') {|f| f.write(new_data) }

  # remove file.css
  File.delete(file)

  puts "\n==> Completed converting #{file}.\n"

end

If you want Scss, just change line 14 from ":sass" to ":scss"