If you want to change menu background color on mobile, Please enter below css in wp-admin/appearance/customise/additionalCSS box.

@media (max-width: 991px) {
.main_menu .navbar {
    background: #000000 !important;
}
}

Please change color code as needed.

Also, after changing menu bg color, if you want to change hamburger menu icon, you can see this article https://kb.bolvo.com/docs/airtech-wordpress-theme/change-menu-bg-color-on-mobile/