.details summary {
    position: relative;
    display: block; /* 矢印を消す */
    padding-left: 20px; /* アイコン分の余白 */
    cursor: pointer;
  }
  .details summary::-webkit-details-marker {
    display: none; /* 矢印を消す */
  }
  /* 疑似要素でアイコンを表示 */
  .details summary:before,
  .details summary:after {
    content: "";
    margin: auto;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
  }
  .details summary:before {
    width: 16px;
    height: 16px;
    border-radius: 4px;
    background-color: #1da1ff;
  }
  .details summary:after {
    left: 6px;
    width: 5px;
    height: 5px;
    border: 4px solid transparent;
    border-left: 5px solid #fff;
    box-sizing: border-box;
    transition: .1s;
  }
  /* オープン時 */
  .details details[open] summary:after {
    transform: rotate(90deg); /* 90度回転 */
    left: 4px;
    top: 5px;
  }