superset2/superset/assets/visualizations/treemap.css

text {
  pointer-events: none;
}

.grandparent text {
  font-weight: bold;
}

rect {
  fill: none;
  stroke: #fff;
}

rect.parent,
.grandparent rect {
  stroke-width: 2px;
}

rect.parent {
    pointer-events: none;
}

.grandparent rect {
  fill: #eee;
}

.grandparent:hover rect {
  fill: #aaa;
}

.children rect.parent,
.grandparent rect {
  cursor: pointer;
}

.children rect.parent {
  fill: #bbb;
  fill-opacity: .5;
}

.children:hover rect.child {
  fill: #bbb;
}