body {
  font-family: Arial, sans-serif;
  text-align: center;
}
header {
  margin-bottom: 100px;
}
h1 {
  background-color: #000; /*背景黒*/
  color: #ffffff; /*文字色白*/
  margin: 0;
  padding: 10px;
  font-size: 18px;
}
.date {
  font-size: 300%;
  font-family: "Times New Roman", Times, serif;
}