modeling-app/src/components/FileTree.module.css

.folder {
  position: relative;
}

.folder::after {
  content: "";
  width: 1px;
  z-index: -1;
  @apply absolute top-0 bottom-0;
  left: calc(var(--indent-line-left, 1rem) + 0.25rem);
  @apply bg-chalkboard-30;
}

:global(.dark) .folder::after {
  @apply bg-chalkboard-80;
}