Hello @twpblackpool,
First of all, thank you for using XStore and for contacting us.
Please add the next code in Theme Options > Theme Custom CSS > Custom CSS for desctop:
.et_b_header-menu .secondary-menu-wrapper .menu > .item-design-mega-menu:before {content: "";height: 430px;width: 70px;display: block;position: absolute;right: -46px;top: 0;}
Regards