html, body {
  height: 100%;
  margin: 0;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}

div {
  text-align: center;
}

button {
    width:400px;
    border-radius:10px;
    background-color:#00ff00;
    font-weight:bold;
    padding:15px;
    font-size:16px;
}