when apply single class binding to mat-card, mat-card class is not binding.
<mat-card cdkDropList [className]="getClassName(item)"><!-- some content--></mat-card>
In this case we wont be able to see class name as mat-card and no style been applied from material.
Just use the standard [class]
binding, e.g.
<mat-card cdkDropList [class]="getClassName(item)">
<!-- some content-->
</mat-card>
Example based on forked example from docs here.