#tree li.folder::before {
  content: "📁";  /* folder icon */
  padding-inline-end: 0.3em;
}
#tree li.folder.open::before {
  content: "📂";  /* open folder icon */
}
#tree li.folder > ul {
  display: none;
  background-color: white;
  background-color: Canvas;
  margin-left: 1em;
}
#tree li.folder.open > ul {
  display: block;
}