SOURCE

function my$(id) {
  return document.getElementById(id);
}
var isOpen = true;
my$('btn').onclick = function() {
  if(isOpen) {
    document.body.style.backgroundColor = 'black';
    this.value = 'on';
    isOpen =false;
  } else {
    document.body.style.backgroundColor = 'white';
    this.value = 'off';
    isOpen = true;
  }
}

<input type="button" id="btn" value="off" id="btn"/>