Ah, tough one because of the mobile header transparency, but try this:
@media (max-width: 768px) {
#mobile-header,
#mobile-header a {
transition: background-color 400ms ease;
transition-delay: 300ms;
}
#mobile-header.toggled,
#mobile-header.toggled a {
transition: background-color 400ms ease;
transition-delay: 0;
}
#mobile-header .main-nav > ul,
#mobile-header .main-nav .sub-menu {
display: block;
max-height: 0;
overflow: hidden;
transition: max-height 400ms ease-in-out;
}
#mobile-header.toggled .main-nav > ul,
#mobile-header .main-nav .sub-menu.toggled-on {
max-height: 1000px;
transition: max-height 400ms ease-in-out;
}
#mobile-header .main-nav li {
float: none;
clear: both;
display: block!important;
}
}