Edit in JSFiddle

<details class="details js-details layer1">
  <summary class="details-summary js-details-summary layer1"><span class="btn"></span>1つ目のアコーディオンのタイトル</summary>
  <div class="details-content js-details-content layer1">
    <details class="details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>A</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">A-a</p>
        <p class="details-content-item layer3">A-b</p>
      </div>
    </details>
    <details class="details js-details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>B</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">B-a</p>
        <details class="details js-details layer3">
          <summary class="details-summary js-details-summary layer3"><span class="btn"></span>B-b</summary>
          <div class="details-content js-details-content layer3">
            <p class="details-content-item layer4">B-b-1</p>
            <p class="details-content-item layer4">B-b-2</p>
          </div>
        </details>
      </div>
    </details>
  </div>
</details>
<details class="details js-details layer1">
  <summary class="details-summary js-details-summary layer1"><span class="btn"></span>2つ目のアコーディオンのタイトル</summary>
  <div class="details-content js-details-content layer1">
    <details class="details js-details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>A</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">A-a</p>
        <p class="details-content-item layer3">A-b</p>
      </div>
    </details>
    <details class="details js-details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>B</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">B-a</p>
        <details class="details js-details layer3">
          <summary class="details-summary js-details-summary layer3"><span class="btn"></span>B-b</summary>
          <div class="details-content js-details-content layer3">
            <p class="details-content-item layer4">B-b-1</p>
            <p class="details-content-item layer4">B-b-2</p>
          </div>
        </details>
      </div>
    </details>
  </div>
</details>
<details class="details js-details layer1">
  <summary class="details-summary js-details-summary layer1"><span class="btn"></span>3つ目のアコーディオンのタイトル</summary>
  <div class="details-content js-details-content layer1">
    <details class="details js-details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>A</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">A-a</p>
        <p class="details-content-item layer3">A-b</p>
      </div>
    </details>
    <details class="details js-details layer2">
      <summary class="details-summary js-details-summary layer2"><span class="btn"></span>B</summary>
      <div class="details-content js-details-content layer2">
        <p class="details-content-item layer3">B-a</p>
        <details class="details js-details layer3">
          <summary class="details-summary js-details-summary layer3"><span class="btn"></span>B-b</summary>
          <div class="details-content js-details-content layer3">
            <p class="details-content-item layer4">B-b-1</p>
            <p class="details-content-item layer4">B-b-2</p>
          </div>
        </details>
      </div>
    </details>
  </div>
</details>

.details {
	&.layer1 {
		border-top: 2px solid #00a5a0;
		border-left: 2px solid #00a5a0;
		border-right: 2px solid #00a5a0;
		&:last-of-type {
			border-bottom: 2px solid #00a5a0;
		} 
	}
}
.details-summary {
	position: relative;
	display: block;
	padding: 20px;
	color: #fff;
	font-size: 24px;
	font-weight: bold;
	text-align: center;
	&.layer1 {
		background-color: #b2e4e2;
	}
	&.layer2 {
		background-color: #7fd2cf;
	}
	&.layer3 {
		background-color: #4cc0bc;
	}
	&:hover {
		cursor: pointer;
		opacity: 0.8;
	}
	.btn {
		position: absolute;
        top: 37%;
        left: 10%;
        width: 18px;
        height: 18px;
        transform-origin: center center;
        transition-duration: 0.2s;

        &:before,
        &:after {
            content: "";
            background-color: #fff;
            border-radius: 10px;
            width: 18px;
            height: 4px;
            position: absolute;
            top: 7px;
            left: 0;
            transform-origin: center center;
        }
        &:before {
            width: 4px;
            height: 18px;
            top: 0;
            left: 7px;
        }
    }
	&.is-active {
		.btn {
			-webkit-transform: rotate(-180deg);
			transform: rotate(-180deg);
			&:before {
            	content: none;
        	}	
		}
    }
}

.details-summary::-webkit-details-marker {
	display: none;
}

.details-content {
	color: #fff;
	font-size: 24px;
	font-weight: bold;
	text-align: center;
}
.details-content-item {
	padding: 20px;
	&.layer3 {
		background-color: #4cc0bc;
	}
	&.layer4 {
		background-color: #19aea9;
	}
	&:hover {
		cursor: pointer;
		opacity: 0.8;
	}
}

/* 以下の記述は不要です */
body {
  margin: 20px;
}

    let accordionDetails = '.js-details';
    let accordionSummary = '.js-details-summary';
    let accordionContent = '.js-details-content';
    let speed = 500;

    $(accordionSummary).each(function() {
      $(this).on("click", function(event) {
        // summaryにis-activeクラスを切り替え
        $(this).toggleClass("is-active");
        // デフォルトの挙動を無効化
        event.preventDefault();
        if ($(this).parent($(accordionDetails)).attr("open")) {
          // アコーディオンを閉じるときの処理
          $(this).nextAll($(accordionContent)).slideUp(speed, function() {
            // アニメーションの完了後にopen属性を取り除く
            $(this).parent($(accordionDetails)).removeAttr("open");
          });
        } else {
          // アコーディオンを開くときの処理
          // open属性を付ける
          $(this).parent($(accordionDetails)).attr("open", "true");
          // いったんdisplay:none;してからslideDownで開く
          $(this).nextAll($(accordionContent)).hide().slideDown(speed);
        }
      })
    })