Check box trick.css

 .body {
     width: 100%;
     height: 100%;
     margin: 0;
     padding: 0;
     font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
     
 }

 .nav {
     border-bottom: 1px dotted dodgerblue;
     text-align: right;
     height: 70px;
     line-height: 70px;
 }

 .menu {
     margin: 0 35px 0 0;
 }

 .menu a {
     clear: right;
     text-decoration: none;
     color: grey;
     margin: 0 10px;
     line-height: 80px;
     cursor: crosshair;
 }

 span {
     color: hotpink;
 }

 .nav label{
     margin: 0 40px 0 0;
     font-size: 30px;
     line-height: 70px;
     display: block;
        cursor: pointer;
        width: 26px;
        float: right;
 }

#toggle {
    display: none;
}
 
@media screen and (max-width: 500px) {}

.menu {
   text-align: center;
    width: 100%;
    display: none;
}

.menu a {
    display: block;
    border: 1px solid dodgerblue;
    border-radius: 50px;
    border-left: 100px;
    margin: 0;
}

#toggle:checked + .menu {
    display: block;
    animation: name duration timing-function delay iteration-count direction fill-mode;
    animation-timing-function: ease-in-out;
    animation-delay: 2s;
    animation-duration: 5s;
    background-image: url(portfolio-2.jpg);
}

.body{
    background-repeat: no-repeat;
     background-size: cover;
}