style.css

*{
  margin:0;
  padding:0;
}

body{
  display:flex;
   position: absolute;
  font-family:Sans-serif;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

nav{
  width:300px;
  display:flex;
  overflow:auto;
  flex-direction:column;
}

header{
  font-size:30px;
 
}

nav header{
  position:fixed;
  padding:20px;
  background-color:#fff;
}

nav ul{
  margin-top:75px;
}

nav ul li{
  padding:20px;
  list-style:none;
  border-top:1px solid;
  
}

main{
  flex:1;
  overflow:auto;
  padding:45px;
}

section{
  margin-bottom:30px;
}

p{
  margin-top:20px;
  margin-bottom:20px;
}

main ul{
  margin-left:60px;
}
main ul li{
 margin-top:20px;
}

.nav-link{
  color:#000;
  text-decoration:none;
  cursor:pointer;
}

code{
 display:block;
 padding:15px;
  margin:15px;
  line-height:2;
  border-radius:7px;
 background-color:#e0e0e0;
  
}

@media(max-width:650px){
  body{
    display:flex;
    flex-direction:column;
    position:relative;
  }
  nav{
    height:250px;
    position:relative;
    
    width:100%;
  }
  nav header{
    position:absolute;
    background-color:#fff;
  }
  nav ul{
        overflow:auto;
    border:1px solid;
  }
  main{
  }
}