gistfile1.rb

require 'cfpropertylist'

raise "#{$0}: Usage: #{$0} SourceSkin.bundle Customisations.bundle TargetSkin.bundle" unless ARGV.count == 3
source, custom, target = ARGV
CONFIG_NAME = "configuration.plist"

def plist_from_path(path)
  CFPropertyList.native_types(CFPropertyList::List.new(:file => path).value) if File.exists?(path)
end

# do it
`cp -R #{source} #{target}` unless source == target
`rsync -av --exclude=**/#{CONFIG_NAME} #{custom}/ #{target}`

Dir["#{custom}/**/#{CONFIG_NAME}"].each do |changes_path|
  rel_path = changes_path[custom.length..-1]
  config_path = File.join(source, rel_path)
  config = plist_from_path config_path
  
  changes = plist_from_path changes_path  
  config.merge! changes
  
  File.open(File.join(target, rel_path), "w") {|f| f.write(config.to_plist) }
end