如何给bootstrap列表组以透明背景？

<div class="list-group transparent-list-group">
  <a href="#" class="list-group-item">Item 1</a>
  <a href="#" class="list-group-item">Item 2</a>
  <a href="#" class="list-group-item">Item 3</a>
</div>

.transparent-list-group {
  background-color: transparent;
}
.transparent-list-group .list-group-item {
  background-color: transparent;
}