Styles

The markup page references an external stylesheet document.

<!DOCTYPE html>
<html>
  <head>
    <title>Our Solar System</title>
    <!-- the link element is used to reference a stylesheet -->
    <link rel="stylesheet" href="styles/solarsystem.css" />
    ...cut...

The CSS file contains style rules for the document (solarsystem.css)

Contents of the CSS file: 
body {
  margin-left: 5%;
  margin-top: 2em;
  margin-right: 5%;
  background-image: linear-gradient(white, #ffffdd);
  background-color: #ffffdd;
  font-family: Calibri, Arial, sans-serif;
  height: 100vh;
}

h1 {
  font-family: Calibri, Arial, sans-serif;
  color: #333333;
  border-bottom: 3px solid #333333;
}

ul.gallery {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-wrap: wrap;
  justify-content: flex-start;
}

ul.gallery li {
  margin-top: 1em;
  font-size: 1.25em;
  height: 250px;
  width: 220px;
  flex-basis: 1 0 auto;
  text-align: center;
  border: thin dotted #333333;
  margin: 10px;
  padding: 10px;
}

ul.gallery li img {
  border: none;
}

footer {
  clear: both;
  margin-top: 2rem;
  padding: 1em;
  font-size: smaller;
}

a:link, a:visited {
  text-decoration: none;
  color: blue;
}

a:hover {
  text-decoration: underline;
}

a:active {
  border: none;
  text-decoration: none;
}