body {
  margin: 0;
}

h1 {
  text-align: center;
  margin: 0;
  padding: 10px;
}

main {
  width: 90%;
  margin: auto;
}

div {
  border: 1px solid black;
  display: inline-block;
  width: 30%;
  padding: 0.5em;
  box-sizing: border-box;
  margin: 1.5%;
  background-color: #F5FFF5;
}

header {
  width: 100%;
  background-color: lightgray;
}

nav {
  width: 100%;
  background-color: black;
  color: white;
}

nav li {
  display: inline-block;
}

nav ul {
  margin: 0;
  text-align: center;
  padding: 0;
}

nav a {
  padding: 1rem;
  display: block;
  text-decoration: none;
  color: white;
  box-sizing: border-box;
}

nav a:hover {
  background-color: red;
}

@media (max-width: 900px) {
  main>div{
    margin-left: 5%;
    margin-right: 5%;
    width: 35%;
    background-color: #FFFFEE;
  }
}
@media (max-width: 600px){
  main>div{
    width: inherit;
    margin-left: 5%;
    margin-right: 5%;
    background-color: #FFF5F5;
  }
    nav{
      width: 25%;
      margin-left: auto;
      margin-right: auto;
  }
    h1{
      text-align: left;
    }
}