d3/examples/choropleth/choropleth.css

svg {
  background: #eee;
  width: 960px;
  height: 500px;
}

#counties path {
  stroke: #fff;
  stroke-width: .25px;
}

#states path {
  fill: none;
  stroke: #fff;
  stroke-width: 1.5px;
}