treegrid.css

#treegrid {
  width: 100%;
  white-space: nowrap;
  border-collapse: collapse;
}

#treegrid th {
  text-align: left;
  background-color: #eee;
}

/* Extra space between columns for readability */
#treegrid th, #treegrid td {
  padding-right: 2ch;
  padding-bottom: 3px;
}

#treegrid :focus {
  background-color: hsl(230, 60%, 40%);
  color: white;
  fill: white; /* For triangle color */
  outline: 0;
}

/* Hide collapsed rows */
#treegrid tr[aria-hidden="true"] {
  display: none;
}

/* Indents */
#treegrid tr[aria-level="2"] > td:first-child {
  padding-left: 2.5ch;
}
#treegrid tr[aria-level="3"] > td:first-child {
  padding-left: 5ch;
}
#treegrid tr[aria-level="4"] > td:first-child {
  padding-left: 7.5ch;
}
#treegrid tr[aria-level="5"] > td:first-child {
  padding-left: 10ch;
}

/* Collapse/expand icons */
#treegrid tr > td:first-child::before {
  font-family: monospace;
  content: " ";
  display: inline-block;
  width: 2ch;
  height: 11px;
  transition: transform .3s;
  transform-origin: 5px 5px;
}

#treegrid tr[aria-expanded] > td:first-child::before {
  /*background: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='12' height='12'><polygon fill='black' points='2,0 2,10 10,5'></polygon></svg>") no-repeat;*/
  background-image: url("row-expand-icon.svg"), url("row-expand-icon-highlighted.svg");
  background-repeat: no-repeat;
}

#treegrid tr[aria-expanded]:focus > td:first-child::before,
#treegrid tr[aria-expanded] > td:focus:first-child::before {
  background-image: url("row-expand-icon-highlighted.svg");
}

#treegrid tr[aria-expanded="true"] > td:first-child::before {
  transform: rotate(90deg);
}