<style>
  .btn {
    color: white;
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    background-color: blue;
  }

  #submit {
    background-color: red;
  }
</style>

<button class="btn">ただのボタン</button>
<button class="btn">ただのボタン</button>
<button class="btn" id="submit">送信ボタン</button>
<button class="btn">ただのボタン</button>