discourse-header-submenus/desktop/desktop.scss

$item-height: 40px;

@if $Fixed_mode == "true" {
  .header-submenus {
    position: sticky;
    top: 0;
    z-index: z("header") + 1;
  }

  .d-header-wrap {
    top: $item-height;
  }
}

.top-menu {
  .menu-item.vmo {
    display: none;
  }
}