el-tree .el-tree-node__expand-icon.expanded {
  -webkit-transform: rotate(0deg);
  transform: rotate(0deg);
}

.el-tree .el-tree-node__expand-icon.expanded {
  -webkit-transform: rotate(0deg);
  transform: rotate(0deg);
}

有子节点 且未展开

.el-tree .el-icon-caret-right:before {
  background: url('~@/assets/tree/add.png') no-repeat 0 3px;
  content: '';
  display: block;
  width: 16px;
  height: 16px;
  font-size: 16px;
  background-size: 16px;
}

有子节点 且已展开

.el-tree .el-tree-node__expand-icon.expanded.el-icon-caret-right:before {
  background: url('~@/assets/tree/minus.png') no-repeat 0 3px;
  content: '';
  display: block;
  width: 16px;
  height: 16px;
  font-size: 16px;
  background-size: 16px;
}

没有子节点

.el-tree .el-tree-node__expand-icon.is-leaf::before {
  background: #fff;
  content: '';
  display: block;
  width: 16px;
  height: 16px;
  font-size: 16px;
  background-size: 16px;
}