zotero/scss/mac/_virtualized-table.scss

.virtualized-table, .drag-image-container {
  .twisty {
    width: 19px;
    
    svg {
      fill: #888;
      width: 16px;
    }
  }

  .focused:not(.highlighted) .twisty svg {
    fill: #fff;
  }

  .spacer-twisty {
    min-width: 19px;
  }
}

.virtualized-table-body, .drag-image-container{
  .cell:not(:first-child) {
    border-inline-start: 1px solid #ddd;
  }
}