#main_icon{
    background-image: url(../img/topic24.png);
    background-position: center center;
    background-repeat: no-repeat;
}

#main_submenu{ display: none; }

@media screen and (max-width: 600px){
    
    #main_nav .nav_button{
        display: none;
    }
    #main_submenu{
        display: block;
    }
    
}