darkness.rb

# detects whether a terminal is dark-on-light or light-on-dark.
# Only purposefully supports Terminal.app and iTerm, but a bunch of things use
# $COLORFGBG.
module BackgroundDetector
  DARK  = :dark
  LIGHT = :light

  def terminal_app
    theme = `/usr/libexec/PlistBuddy -c "Print :'Default Window Settings'" ~/Library/Preferences/com.apple.Terminal.plist`.chomp
    blob = `/usr/libexec/PlistBuddy -c "Print :'Window Settings':'#{theme}':BackgroundColor" ~/Library/Preferences/com.apple.Terminal.plist`.chomp

    md = blob.force_encoding(Encoding::BINARY).match(/([01]\.\d+) ([01]\.\d+) ([01]\.\d+)/m)
    r = md[1].to_f
    g = md[2].to_f
    b = md[3].to_f

    avg = (r + g + b) / 3.0

    avg < 0.5 ? DARK : LIGHT
  end

  def call
    if fgbg = ENV['COLORFGBG']
      parts = fgbg.split(';')
      fg = parts[0].to_i
      bg = parts[1].to_i
      fg <= 6 || bg == 8 ? DARK : LIGHT
    else
      terminal_app
    end
  end
end

Oh man, hahah nice.