div.tooltip {
  position: absolute;
  text-align: center;
  width: 60px;
  height: 28px;
  padding: 2px;
  font: 12px sans-serif;
  background: lightsteelblue;
  border: 0px;
  border-radius: 8px;
  pointer-events: none;
}

circle.node {
  stroke: #fff;
  stroke-width: 1.5px;
}

line.link {
  stroke: #999;
  stroke-opacity: .6;
}