However, here is sample code for you that you can put in your custom CSS to change the close icon:

.offcanvas-active .burger-icon>span, #modal-menu-toggler.active .burger-icon>span {
    background-color: red;
}

Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards