m4.md

define(`red', `#f00')
define(`blue', `#0f0')

#foo {
    background:red();
    color:blue();
}

m4 foo.css

#foo {
    background:#f00;
    color:#0f0;
}

  __  ___ ______ ______    _____  _____ _____ 
 /_ |/ _ \____  |____  |  / ____|/ ____/ ____|
  | | (_) |  / /    / /  | |    | (___| (___  
  | |\__, | / /    / /   | |     \___ \\___ \ 
  | |  / / / /    / /    | |____ ____) |___) |
  |_| /_/ /_/    /_/      \_____|_____/_____/