summary {
  list-style: none;
  cursor: pointer;
}
summary::-webkit-details-marker {
  display: none;
}

/* ＋アイコン */
summary::before {
  content: "+";
  display: inline-block;
  margin-right: 8px;
  font-weight: bold;
  font-size: 18px;
}

/* 開いたら − に変わる */
details[open] > summary::before {
  content: "−";
}

/* デザイン */
details {
  border: 1px solid #1A1A1A;
  margin-bottom: 8px;
}
summary {
  padding: 10px;
  font-weight: bold;
}
details > div {
  padding: 10px;
  border-top: 1px solid #1A1A1A;
}