pre {
  margin: 0 auto;
  padding: 10px;
  background: lightcyan;
  text-align: center;
}
.center {
  text-align: center;
  padding: 10px;
}
.center button {
  border: 3px solid green;
  background: white;
  border-radius: 10px;
  padding: 10px;
}
.container {
  margin: 0 auto;
  max-width: 500px;
  width: 50%;
}