Hi Annelene,
Thanks for reaching out.
It seems that you are using the default header and you need to add the custom CSS code into the Theme Options > CSS to add background color to the mobile menu only.
.x-nav-wrap.mobile
{
background-color:#ffffff;
}
Please remember that the above code will work if copied as it is and don’t conflict with any existing style.
Please note that the code provided serves only as a guide to help you get started custom coding on your own if there’s no option offered in our theme or the products we bundle.
We really do not provide support for custom codes that means we can’t fix it in case it conflicts with something on your site nor will we enhance it.
Thanks