svg {
  background: white;
}
.node {
  fill: #fff;
  stroke: steelblue;
  stroke-width: 1.5px;
}
.node:hover {
  fill: steelblue;
}
.internal.node {
  stroke: #A55;
}
.internal.node:hover {
  fill: #A55;
}

.link, .root_link {
  fill: none;
  stroke: #ccc;
  stroke-width: 1.2px;
}