I have tried this custom css, however nothing has changed and the search icon isn’t appearing.

@media only screen and (max-width: 767px) {
.responsive #header .main_menu ul, .main_menu, #header_main_alternate {
display: block;
}
#top #menu-item-search {
display: block;
margin-top: -90px;
}
.av-main-nav > li { display: none; }}

Any suggestions?