Here you go, white background for the transparent menu smaller than 991 pix with black text:
(adjust accordingly)
@media (max-width: 991px)
{.navbar .nav {
background: #ffffff;
}}
.collapse.show {
display: block;
background: #ffffff;
}
@media (max-width: 991px)
{
.navbar.navbar-header-wrap .nav .menu-item .nav-link, .navbar.navbar-header-wrap .nav a.cart-icon, .navbar.navbar-header-wrap .nav .theme-search-block a {
color: #000;
}}
@media (max-width: 991px)
{.navbar.navbar-header-wrap .nav .dropdown-item {
color: #000000;
}}