One possible solution is to use purely CSS, so there's no need for for hacks like `layerList.classList.add("iconLayerList")`.

The below CSS works for the above simple case (more complex selector might be necessary with complex maps):
```
div[title=Layers] calcite-icon {
  background-image: url('https://i.imgur.com/ESUiDwT.png');
  background-repeat: no-repeat;
  background-size: 20px;
  background-position: center;
}
```
JS part stays unchanged:
```
const layerListExpand = new Expand({
    view: view,
    expandTooltip: "Layers",
    expandIcon: '',
    content: new LayerList({
      view: view,
    })
});

view.ui.add(layerListExpand, "top-right");
```