app/node_modules/@angular/material/dialog/_dialog-legacy-padding.scss

// Legacy padding for the dialog. Copied from the non-MDC dialog styles.
$legacy-padding: 24px !default;

/// Mixin that applies creates styles for MDC-based dialog's to have legacy outer
/// padding. By default, the dialog does not have any padding. The individual directives
/// such as `matDialogContent`, `matDialogActions` or `matDialogTitle` set the padding.
@mixin legacy-padding() {
  // Sets the outer padding for the projected dialog user-content.
  .mat-mdc-dialog-surface {
    padding: $legacy-padding;
  }

  // Updates the MDC title, content and action elements to account for the legacy outer
  // padding. The elements will be shifted so that they behave as if there is no margin.
  // This allows us to still rely on MDC's dialog spacing while maintaining a default outer
  // padding for backwards compatibility.
  .mat-mdc-dialog-container {
    .mat-mdc-dialog-title {
      margin-top: -$legacy-padding;
    }

    .mat-mdc-dialog-actions {
      margin-bottom: -$legacy-padding;
    }

    .mat-mdc-dialog-title, .mat-mdc-dialog-content, .mat-mdc-dialog-actions {
      margin-left: -$legacy-padding;
      margin-right: -$legacy-padding;
    }
  }
}