angular material expansion toggle icon styles code example
Example: mat-panel toggle icon
<mat-panel-description>
<mat-icon *ngIf="!panelOpenState">add</mat-icon>
<mat-icon *ngIf="panelOpenState">remove</mat-icon>
</mat-panel-description>
<mat-panel-description>
<mat-icon *ngIf="!panelOpenState">add</mat-icon>
<mat-icon *ngIf="panelOpenState">remove</mat-icon>
</mat-panel-description>