import {ViewEncapsulation} from '@angular/core';
@Component({
.....,
encapsulation: ViewEncapsulation.None
})
.mat-dialog-container {
padding: 0px !important;
}
.custom-dialog-container .mat-dialog-container {
/* add your styles */
}
this.dialog.open(MyDialogComponent, { panelClass: 'custom-dialog-container' })