.az {
  width:6vmin;
  height:5vmin;
  border:thin solid;
  font-size:4vmin;
  text-align:center;
  background-color:palegreen;
  margin:.2vmin;
}
key_clicked function in clicks.js
function key_clicked(el) {
  var key=el.textContent;
  var square=document.getElementById(g_square_id);
  square.textContent=key;
  var a=square.getAttribute('a');
  if (a == key) {
    square.style.color='black';
  } else {
    square.style.color='red';
  }
}

You should now have a fully operational game so it's time to turn our attention to the final layout.

Here's my map for you to download.