topic-tree.css

.node circle {
  cursor: pointer;
  fill: #fff;
  stroke: steelblue;
  stroke-width: 1.5px;
}

.node text {
  font-size: 11px;
}

path.link {
  fill: none;
  stroke: #ccc;
  stroke-width: 1.5px;
}

.popup rect {
  fill: blue;
  stroke: black;
  stroke-width: 1.5px;
  opacity: 1;
  fill-opacity: 1;
}

.popup text {
   fill: red;
   font-weight: bold; 
}