No theme
<ul id="accordionExample">
<li data-toggle="collapse" data-target="#collapseOne">
Lvl 1
<ul id="collapseOne" class="collapse show" data-parent="#accordionExample">
<li>Lvl 1.1</li>
<li>Lvl 1.2</li>
</ul>
</li>
<li data-toggle="collapse" data-target="#collapseTwo">
Lvl 2
<ul id="collapseTwo" class="collapse" data-parent="#accordionExample">
<li>Lvl 2.1</li>
<li>Lvl 2.2</li>
</ul>
</li>
</ul>
Dashboard theme example
<ul id="accordionExample" class="nav flex-column">
<li class="nav-item" data-toggle="collapse" data-target="#collapseOne">
<a class="nav-link" href="#">
Lvl 1
</a>
<ul id="collapseOne" class="collapse show" data-parent="#accordionExample">
<li>Lvl 1.1</li>
<li>Lvl 1.2</li>
</ul>
</li>
<li class="nav-item" data-toggle="collapse" data-target="#collapseTwo">
<a class="nav-link" href="#">
Lvl 2
</a>
<ul id="collapseTwo" class="collapse" data-parent="#accordionExample">
<li>Lvl 2.1</li>
<li>Lvl 2.2</li>
</ul>
</li>
</ul>