index.css

html {
    height: 100%;
    width: 100%;
    font-family: sans-serif;
}

body {
    margin: 0;
}

nav {
    display: none;
}

a.navicon > img {
    outline: 3px solid navy;
    background-color: whitesmoke;
    height: 12.5vw;
    width: 12.5vw;
}

a.navicon:hover+nav, nav:hover {
    display: block;
}

ul {
    list-style: none inside;
    margin: 0;
    padding: 0;
}

ul.mainmenu > li {
    font-size: 2.5em;
    text-indent: 1em;
    background-color: bisque;
}

ul::after {
	clear: both;
	content: "";
	display: table;
}

div {
    text-align: center;
}

#secondJoke, #thirdJoke {
    margin-top: 100px;
}

#buttons {
    margin-top: 100px;
}

@media only screen and (min-width: 600px) {
     
    a.navicon {
        display: none;
    }
    
    ul.mainmenu > li {
        float: left;
        width: 50%;
        font-size: 1.5em;
        text-align: center;
    }

    ul.mainmenu {
        width: 100%;
    }

    nav {
        width: 100%;
        display: block;
    }
}