.container {
  display: grid;
  place-content: center;
  place-items: center;
}

h1 {
	margin: 3rem;
}
div.navi {
	margin: 1rem;
}

.flat.border {
  position: relative;
  text-align: center;
  /* 中央揃え */
  display: block;
  margin: 0 auto;
  /* 文字サイズ */
  font-size: 2rem;
  /* 幅の指定 */
  width: 100%;
  max-width: 28rem;
  box-sizing: border-box;
  padding: 10px;
  /* 色の指定 */
  /*background: linear-gradient(135deg, #ffd900, #ff9300);*/
  color: #fff;
  font-weight: bold;
  border-radius: 8px;
  transition: 0.3s;
  border: 2px solid #33ff00;
  background: #fff;
  color: #339900;
}
.flat.border:hover {
  background: #33ff00;
  color: #fff;
}
.flat.border:active, .flat.border:focus {
  top: 2px;
}

#footer {
margin-top: 3rem;
width: 100%;
}