gnome-shell/data/theme/gnome-shell-sass/widgets/_notifications.scss

/* Notifications & Message Tray */

$notification_banner_height: 64px;
$notification_banner_width: 34em;

// Banner notifications
.notification-banner {
  min-height: $notification_banner_height;
  width: $notification_banner_width;
  box-shadow: 0 2px 4px 2px $shadow_color;
  border-radius: $modal_radius;
  margin: $base_margin;

  @if $contrast == 'high' {
    @include draw_hc_inset();
  }

  .notification-actions {
    spacing: 0;
  }

  .notification-button {
    @extend %bubble_button;
  }
}