<p>フォントサイズ5vw</p>

* {
  background-color: #3b3f47;
  font-family: "游ゴシック", "YuGothic", "Yu Gothic", 'ヒラギノ角ゴ ProN W3', "Hiragino Kaku Gothic ProN", "Meiryo", Helvetica, Arial, sans-serif;
  height: 100%;
  text-align: center;
}

p {
  font-size: 5vw;
  font-weight: bold;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 0;
  margin: 0;
  color: white;
}

@media(min-width:700px) {
  p {
    font-size:  35px;
  }
}