style.css

html {
  scroll-behavior: smooth;
}

body {
  display: flex;
  font-family: 'Helvetica', 'Arial', sans-serif;
}

section {
  line-height: 1.5em;
}

nav {
  width: 320px;
  height: 100%;
  display: flex;
  flex-direction: column;
  position: fixed;
  overflow: auto;
}

nav > a {
  color: black;
  margin-right: 10px;
  text-decoration: none;
  border-top: 1px solid black;
  border-right: 1px solid black;
  border-left: 1px solid black;
  padding: 10px;
  transition: background-color 1s;
}

nav > a:hover {
  background-color: silver;
}

a:nth-child(n+6) {
    border-bottom: 1px solid black;
}

main {
  margin-left: 340px;
}

code {
  display: inline-block;
  padding: 10px;
  background-color: silver;
}

@media (max-width: 640px) {
  body {
    display: flex;
    flex-direction: column;
  }
  
  nav {
    width: 100%;
    height: 200px;
    position: relative;
    overflow: auto;
  }
  
  main {
    margin-left: auto;
  }
}