Consistent Styling for Nested Lists with Bootstrap

This was my approach:

    .list-group-collapse li > ul li:first-child {
        border-top-left-radius: 0;
        border-top-right-radius: 0;
    }

    .list-group-collapse li > ul {
        margin-left: -16px;
        margin-right: -16px;
        margin-bottom: -11px;
    }

If you are using BS3, then adding the .list-group-collapse class to your grouped list will do the trick. See JSFiddle: https://jsfiddle.net/oscar_dr/d2wpn8sd/1/

Of course, you could extend BS with this class or change the values to your custom measures if you have a customized Bootstrap.

EDIT: Added snippet to answer:

.list-group-collapse li>ul li:first-child {
  border-top-left-radius: 0;
  border-top-right-radius: 0;
}

.list-group-collapse li>ul {
  margin-left: -16px;
  margin-right: -16px;
  margin-bottom: -11px;
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" />

<div class="container">
  <div class="row">
    <div class="col-xs-6">
      <h4>
        With collapse
      </h4>
      <hr>
      <ul class="list-group list-group-collapse">
        <li class="list-group-item">
          <h3>
            Level 1
          </h3>

          <ul class="list-group">
            <li class="list-group-item">
              <h4>
                Level 2.1
              </h4>

              <ul class="list-group">
                <li class="list-group-item">
                  Item 2.1.1
                </li>
                <li class="list-group-item">
                  Item 2.1.2
                </li>
                <li class="list-group-item">
                  Item 2.1.3
                </li>
              </ul>
            </li>

            <li class="list-group-item">
              <h4>
                Level 2.2
              </h4>
            </li>
          </ul>
        </li>
      </ul>
    </div>

    <div class="col-xs-6">
      <h4>
        Without collapse
      </h4>
      <hr>
      <ul class="list-group">
        <li class="list-group-item">
          <h3>
            Level 1
          </h3>

          <ul class="list-group">
            <li class="list-group-item">
              <h4>
                Level 2.1
              </h4>

              <ul class="list-group">
                <li class="list-group-item">
                  Item 2.1.1
                </li>
                <li class="list-group-item">
                  Item 2.1.2
                </li>
                <li class="list-group-item">
                  Item 2.1.3
                </li>
              </ul>
            </li>

            <li class="list-group-item">
              <h4>
                Level 2.2
              </h4>
            </li>
          </ul>
        </li>
      </ul>
    </div>
  </div>
</div>

Nested Group Lists

Nested Group Lists

.just-padding {
  padding: 15px;
}

.list-group.list-group-root {
  padding: 0;
  overflow: hidden;
}

.list-group.list-group-root .list-group {
  margin-bottom: 0;
}

.list-group.list-group-root .list-group-item {
  border-radius: 0;
  border-width: 1px 0 0 0;
}

.list-group.list-group-root > .list-group-item:first-child {
  border-top-width: 0;
}

.list-group.list-group-root > .list-group > .list-group-item {
  padding-left: 30px;
}

.list-group.list-group-root > .list-group > .list-group > .list-group-item {
  padding-left: 45px;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="just-padding">

<div class="list-group list-group-root well">
  
  <a href="#" class="list-group-item">Item 1</a>
  <div class="list-group">
    
    <a href="#" class="list-group-item">Item 1.1</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 1.1.1</a>
      <a href="#" class="list-group-item">Item 1.1.2</a>
      <a href="#" class="list-group-item">Item 1.1.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 1.2</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 1.2.1</a>
      <a href="#" class="list-group-item">Item 1.2.2</a>
      <a href="#" class="list-group-item">Item 1.2.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 1.3</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 1.3.1</a>
      <a href="#" class="list-group-item">Item 1.3.2</a>
      <a href="#" class="list-group-item">Item 1.3.3</a>
    </div>
    
  </div>
  
  <a href="#" class="list-group-item">Item 2</a>
  <div class="list-group">
    
    <a href="#" class="list-group-item">Item 2.1</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 2.1.1</a>
      <a href="#" class="list-group-item">Item 2.1.2</a>
      <a href="#" class="list-group-item">Item 2.1.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 2.2</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 2.2.1</a>
      <a href="#" class="list-group-item">Item 2.2.2</a>
      <a href="#" class="list-group-item">Item 2.2.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 2.3</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 2.3.1</a>
      <a href="#" class="list-group-item">Item 2.3.2</a>
      <a href="#" class="list-group-item">Item 2.3.3</a>
    </div>
    
  </div>
  
  
  <a href="#" class="list-group-item">Item 3</a>
  <div class="list-group">
    
    <a href="#" class="list-group-item">Item 3.1</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 3.1.1</a>
      <a href="#" class="list-group-item">Item 3.1.2</a>
      <a href="#" class="list-group-item">Item 3.1.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 3.2</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 3.2.1</a>
      <a href="#" class="list-group-item">Item 3.2.2</a>
      <a href="#" class="list-group-item">Item 3.2.3</a>
    </div>
    
    <a href="#" class="list-group-item">Item 3.3</a>
    <div class="list-group">
      <a href="#" class="list-group-item">Item 3.3.1</a>
      <a href="#" class="list-group-item">Item 3.3.2</a>
      <a href="#" class="list-group-item">Item 3.3.3</a>
    </div>
    
  </div>
  
</div>
  
</div>

View on fiddler: https://jsfiddle.net/u3gd85cj/


Nested Group Lists (Collapsable)

Nested Group Lists (Collapsable)

$(function() {
        
  $('.list-group-item').on('click', function() {
    $('.glyphicon', this)
      .toggleClass('glyphicon-chevron-right')
      .toggleClass('glyphicon-chevron-down');
  });

});
.just-padding {
  padding: 15px;
}

.list-group.list-group-root {
  padding: 0;
  overflow: hidden;
}

.list-group.list-group-root .list-group {
  margin-bottom: 0;
}

.list-group.list-group-root .list-group-item {
  border-radius: 0;
  border-width: 1px 0 0 0;
}

.list-group.list-group-root > .list-group-item:first-child {
  border-top-width: 0;
}

.list-group.list-group-root > .list-group > .list-group-item {
  padding-left: 30px;
}

.list-group.list-group-root > .list-group > .list-group > .list-group-item {
  padding-left: 45px;
}

.list-group-item .glyphicon {
  margin-right: 5px;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="just-padding">

<div class="list-group list-group-root well">
  
  <a href="#item-1" class="list-group-item" data-toggle="collapse">
    <i class="glyphicon glyphicon-chevron-right"></i>Item 1
  </a>
  <div class="list-group collapse" id="item-1">
    
    <a href="#item-1-1" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 1.1
    </a>
    <div class="list-group collapse" id="item-1-1">
      <a href="#" class="list-group-item">Item 1.1.1</a>
      <a href="#" class="list-group-item">Item 1.1.2</a>
      <a href="#" class="list-group-item">Item 1.1.3</a>
    </div>
    
    <a href="#item-1-2" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 1.2
    </a>
    <div class="list-group collapse" id="item-1-2">
      <a href="#" class="list-group-item">Item 1.2.1</a>
      <a href="#" class="list-group-item">Item 1.2.2</a>
      <a href="#" class="list-group-item">Item 1.2.3</a>
    </div>
    
    <a href="#item-1-3" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 1.3
    </a>
    <div class="list-group collapse" id="item-1-3">
      <a href="#" class="list-group-item">Item 1.3.1</a>
      <a href="#" class="list-group-item">Item 1.3.2</a>
      <a href="#" class="list-group-item">Item 1.3.3</a>
    </div>
    
  </div>
  
  <a href="#item-2" class="list-group-item" data-toggle="collapse">
    <i class="glyphicon glyphicon-chevron-right"></i>Item 2
  </a>
  <div class="list-group collapse" id="item-2">
    
    <a href="#item-2-1" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 2.1
    </a>
    <div class="list-group collapse" id="item-2-1">
      <a href="#" class="list-group-item">Item 2.1.1</a>
      <a href="#" class="list-group-item">Item 2.1.2</a>
      <a href="#" class="list-group-item">Item 2.1.3</a>
    </div>
    
    <a href="#item-2-2" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 2.2
    </a>
    <div class="list-group collapse" id="item-2-2">
      <a href="#" class="list-group-item">Item 2.2.1</a>
      <a href="#" class="list-group-item">Item 2.2.2</a>
      <a href="#" class="list-group-item">Item 2.2.3</a>
    </div>
    
    <a href="#item-2-3" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 2.3
    </a>
    <div class="list-group collapse" id="item-2-3">
      <a href="#" class="list-group-item">Item 2.3.1</a>
      <a href="#" class="list-group-item">Item 2.3.2</a>
      <a href="#" class="list-group-item">Item 2.3.3</a>
    </div>
    
  </div>
  
  
  <a href="#item-3" class="list-group-item" data-toggle="collapse">
    <i class="glyphicon glyphicon-chevron-right"></i>Item 3
  </a>
  <div class="list-group collapse" id="item-3">
    
    <a href="#item-3-1" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 3.1
    </a>
    <div class="list-group collapse" id="item-3-1">
      <a href="#" class="list-group-item">Item 3.1.1</a>
      <a href="#" class="list-group-item">Item 3.1.2</a>
      <a href="#" class="list-group-item">Item 3.1.3</a>
    </div>
    
    <a href="#item-3-2" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 3.2
    </a>
    <div class="list-group collapse" id="item-3-2">
      <a href="#" class="list-group-item">Item 3.2.1</a>
      <a href="#" class="list-group-item">Item 3.2.2</a>
      <a href="#" class="list-group-item">Item 3.2.3</a>
    </div>
    
    <a href="#item-3-3" class="list-group-item" data-toggle="collapse">
      <i class="glyphicon glyphicon-chevron-right"></i>Item 3.3
    </a>
    <div class="list-group collapse" id="item-3-3">
      <a href="#" class="list-group-item">Item 3.3.1</a>
      <a href="#" class="list-group-item">Item 3.3.2</a>
      <a href="#" class="list-group-item">Item 3.3.3</a>
    </div>
    
  </div>
  
</div>
  
</div>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>

View on fiddler: https://jsfiddle.net/ann7tctp/