* {
  margin: 0px;
  padding: 0px;

  box-sizing: border-box;

  font-family:
    system-ui,
    -apple-system,
    BlinkMacSystemFont,
    "Segoe UI",
    Roboto,
    Oxygen,
    Ubuntu,
    Cantarell,
    "Open Sans",
    "Helvetica Neue",
    sans-serif;
}

body,
html {
  width: 100vw;
  height: 100vh;

  background-color: lightgrey;
}

header {
  background-color: gray;

  padding: 10px;

  color: white;

  display: flex;
  flex-flow: row wrap;
  justify-content: space-between;
}

header nav ul li {
  display: inline-block;
  padding: 10px;
  margin-top: 5px;

  background-color: black;
  border-radius: 5px;
}

header nav ul li:hover {
  background-color: white;
  color: black;
}

@media screen and (max-width: 518px) {
  header {
    flex-flow: column;
  }

  header nav ul li {
    text-align: center;
    display: block;
  }
}

main {
  display: flex;
  justify-content: center;
}

main section {
  background-color: white;
  width: 95%;

  max-width: 600px;

  margin: 10px;

  border-radius: 10px;
  padding: 10px;
}