zotero/scss/components/_item-tree.scss

#zotero-items-pane {
  min-width: 290px;
  min-height: 150px;
  height: 150px;
  width: 290px;
  
  .items-tree-message {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
  }
}
#zotero-items-tree {
  .virtualized-table-header .icon {
    width: 13px;
    height: 13px;
  }
  
  .cell.primary {
    .retracted {
      width: 12px;
      margin-inline-start: 3px;
    }
    
    .tag-swatch {
      display: inline-block;
      min-width: 8px;
      min-height: 8px;
      margin-inline-start: 3px;
      border-radius: 1px;
    }
  }
  
  .cell.hasAttachment {
    box-sizing: content-box;
    padding: 0 4px;
  }
  
  .cell.numNotes {
    text-align: center;
  }
}