additions to styles.css
#buttons {
  display:flex;
  justify-content: center;
}
#buttons div {
  font-size:3vmin;
  font-family: sans-serif;
  color:ivory;
  padding:1vmin 2vmin;
  margin:1vmin;
  background-color: brown;
}

And we're going to need a new global g_running.

You might enjoy the challenge of trying to get it all working - I know I did!.

Click here to see my attempt in action.

setup()
function setup() {
  clearTimeout(g_timer);
  g_side=45*vmin();
  g_running=false;
  Hide('cont'); Hide('stop'); Show('run');
  var buttons=document.getElementById('buttons');
  buttons.style.width=g_side;
...
}