Guard + sass + autoprefixer

Gemfile

source 'https://rubygems.org/'

gem 'guard'
gem 'guard-sass', git: 'git@github.com:alexdunae/guard-sass.git'
gem 'autoprefixer-rails'

Guardfile

require 'autoprefixer-rails'

guard :sass, {
  input: 'www/wp-content/themes/example/css',
  output: 'www/wp-content/themes/example',
  style: :compressed,
  smart_partials: true,
  all_on_start: true
} do
  callback(:run_on_changes_end) do |_, _, files|
    Array(files).each do |file|
      time = Benchmark.realtime { autoprefix_file(file) }
      benchmark = "[\e[33m%2.2fs\e[0m] " % time
      ::Guard::UI.info("\t\e[1;37mAutoprefixer\e[0m %s%s" % [benchmark, file])
    end
  end
end

def autoprefix_file(file)
  original_css = File.read(file)
  File.open(file, 'w') do |io|
    io << ::AutoprefixerRails.process(original_css, browsers: ['> 1%', 'ie >= 7'])
  end
end