body {
  font-family: Arial, sans-serif;
  background: #f5f5f5;
  margin: 0;
  padding: 0;
  display:table-cell; 
  text-align:right; 
  vertical-align:middle;
  display:flex; 
  justify-content:center;
}

h2 {
  text-align: center;
}


input[readonly] {
  background-color: #e9e9e9;
  color: #555;
}

button {
  padding: 10px;
  background: green;
  color: white;
  border: none;
  width: 100%;
  border-radius: 4px;
  cursor: pointer;
  margin-top: 10px;
}

button:hover {
  background: darkgreen;
}

#mensagem {
  margin-top: 10px;
  font-weight: bold;
  text-align: center;
}

td div {
  display: flex;
  justify-content: center; /* centraliza horizontal */
  align-items: center;     /* centraliza vertical */
  height: 100%;            /* ocupa a célula toda */
}