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.