style.css

.link {
  stroke: #ccc;
}

.node {
  stroke: #fff;
}

.d3-tip {
  line-height: 1;
  font-weight: bold;
  padding: 12px;
  background: rgba(0, 0, 0, 0.8);
  color: #fff;
  border-radius: 2px;
}
/* Creates a small triangle extender for the tooltip */
.d3-tip:after {
  box-sizing: border-box;
  display: inline;
  font-size: 10px;
  width: 100%;
  line-height: 1;
  color: rgba(0, 0, 0, 0.8);
  content: "\25BC";
  position: absolute;
  text-align: center;
}

/* Style northward tooltips differently */
.d3-tip.n:after {
  margin: -1px 0 0 0;
  top: 100%;
  left: 0;
}

/* Style eastward tooltips differently */
.d3-tip.e:after {
  margin: -4px 0 0 0;
  top:50%;
  left: -8px;
}

/* Style southward tooltips differently */
.d3-tip.s:after {
  margin: 0 0 1px 0;
  top:-8px;
  left: 0;
  text-align: center;
}

/* Style westward tooltips differently */
.d3-tip.w:after {
  margin: -4px 0 0 -1px;
  top:50%;
  left: 100%;
}
/*
#footer {
  position: absolute;
  width: 100%;
  clear: both;
  bottom: 0;
  padding: 0;
  margin: 0;
  height: 30px;
  background: #333;
}*/