How to apply some style to the table row in angular material table

Your last mat-header-row has missing some code and use row.isSeen which contains the current iteration row in it.

<tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
<tr mat-row *matRowDef="let row; columns: displayedColumns;" [ngClass]="{'highlight': row.isSeen }"></tr>

Replace last mat-header-row with above code and will work:

Here is the Online_Demo