MediaWiki:CollapsibleTree.css

.collapsible-tree > ul {
  margin: 0;
  padding-left: 25px;
  list-style: none;
  line-height: 2em;
}

.collapsible-tree > ul li {
  position: relative;
}

.collapsible-tree > ul li a {
  padding-left: 0.5rem;
}

.collapsible-tree > ul li::before,
.collapsible-tree > ul li::after {
  content: "";
  position: absolute;
  left: -15px;
  border-left: 1px dashed #999;
}

.collapsible-tree > ul li::before {
  top: 0;
  width: 15px;
  height: 1em;
  border-bottom: 1px dashed #999;
}

/* Hide vertical line for first item */
.collapsible-tree > ul > li:first-child::before {
  border-left: none;
}

.collapsible-tree > ul li::after {
  top: 1.1em;
  bottom: 1px;
}

/* Hide lines for last item */
.collapsible-tree > ul li:last-child::after {
  display: none;
}

/* Toggle icon (inserted via JS) */
.js-toggle-icon {
  position: relative;
  z-index: 1;
  display: inline-block;
  width: 14px;
  margin: 0 2px 0 -23px;
  line-height: 14px;
  text-align: center;
  cursor: pointer;
  font-style: normal;
  background-color: #fff;
  border: 1px solid #666;
  border-radius: 2px;
}