To change hamburger menu on mobile, Please enter below css in wp-admin/appearance/customise/additionalCSS box.
@media (max-width: 991px) { .navbar-light .navbar-toggler span:before { background: #000; } .navbar-light .navbar-toggler span { background: #000; } .navbar-light .navbar-toggler span:after { background: #000; } .navbar-light .navbar-toggler.collapsed span { background: #000; } }
Please modify color code as needed.