Hello,

1. The below code will help you to increase the size of the burger icon.

.wd-header-mobile-nav .wd-tools-icon:before {
    font-size: 29px;
}

2. Next one is for the changing mobile menu background.

.mobile-nav {
    background-color: #000;
}

Both customs add to the “Custom CSS for mobile” area in Theme Settings -> Custom CSS.

Kind Regards