mat-card, how to change default padding

dotless: .mat-card --> mat-card

mat-card { margin: 5px; padding: 5px;}

You must scope them. Angular will take css of its own component first. So if you want to change globally scope them with any class name such as 'custom'.

<mat-card class="custom"></mat-card>

In stylesheet

.custom.mat-card{
  //your styles
 }

To know more, refer to angular material customizing