#top-menu {
    background-color: red;
}

.dropdown_menu_content {
    display: none;
    position: absolute;
    z-index: 10;
    box-shadow: 0 4px 10px 0 rgba(0,0,0,0.2),0 4px 20px 0 rgba(0,0,0,0.19);
}

.platform_dropdown_menu:hover > .dropdown_menu_content {
    display: inline-block;
}