style.css

/* Set the background color of the
 * body to blue and text color to
 * white
 */
body {
  background: blue;
  color: white;
  font-family: sans-serif;
  max-width: 40em;
  margin: 1em;
}

/* Style all articles */
article {
  background: lightgrey;
  color: black;
  padding: 1em;
  border-radius: 1em;
}

/* Add padding of 1em to all direct
 * children of article elements
 */
article>* {
  padding: 1em;
}

/* Style all sections */
section {
  background: white;
  color: black;
  border-radius: 0.5em;
  margin-top: 0.5em;
  padding: 1em;
}

article>h1 {
  margin: 0;
  padding: 0 0 0 1rem;
}

/* Style all h1 elements that are
 * children of section elements
 */
section>h2,
section>h3 {
  margin: 0;
}

/* All elements that we deem important
 * can be rendered differently using this
 * rule - just add...
 *   class="important"
 * ...to the element.
 */
.important {
  background: #CCFF00;
}

/* The first section should have a black border, for no reason
 * other than "because we can"
 */
#rich1 {
  border: 0.25em solid black;
}

/* make navigation links look like the typical blue 
 * buttons seen all over the web these days.
 */
nav a {
  background: #2196F3;
  color: white;
  padding: 0.5em;
  margin: 0.25em;
  border-radius: 0.25em;
  border: white solid 0.1em;
  text-decoration: none;
}

/* This is a pseudo selector.  The element
 * it selects is different based on the
 * contents of the URL fragment after 
 * the (#) hash - so when a user clicks 
 * on a nav link the target will change.
 */
:target {
  background: #FFCC00;
}

/* The default layout is for mobile phones
 * with a nav menu appearing as a column
 * of text.
 */
nav {
  display: flex;
  flex-direction: column;
}


/* Remove the margin from below paragraphs so we
 * don't have extra white space in the boxes.
 */
p {
  margin-bottom: 0;
}

footer {
  text-align: center;
}

/* Media query for screens wider than 30em */
@media (width>42em) {

  /* We change the background - this isn't
   * really necessary but it helps us know
   * when we are changing between styles
   */
  body {
    background: black;
    margin: 1em auto;
  }

  /* On wider screens we make the nav menu
   * a horizontal row
   */
  nav {
    flex-direction: row;
  }
}