<ul>
  <li>Lorem, ipsum.</li>
  <li>Eum, consequuntur?</li>
  <li>Dolores, necessitatibus.</li>
  <li>Cupiditate, eveniet?</li>
  <li>Deserunt, libero!</li>
  <li>Perspiciatis, facere.</li>
  <li>Dolor, minima.</li>
  <li>Iste, sapiente.</li>
  <li>Iusto, ratione?</li>
  <li>Recusandae, autem.</li>
</ul>
body {
  display: grid;
  place-content: center;
  margin: 0;
  min-height: 100vh;
  background-color: hsl(222, 99%, 18%);
  font-family: sans-serif;
  color: hsl(222, 99%, 90%);
}

ul {
  list-style: none;
  padding: 0;
}

li {
  margin-bottom: 0.3rem;
  padding: 0.5rem;
}

/* Select every 3th item in the list. */
li:nth-child(3n) {
  background: hsl(222, 99%, 80%);
  color: hsl(222, 99%, 18%);
}

/* Select every 4th item in the list. */
/* li:nth-child(4n) {
  background: hsl(222, 99%, 80%);
  color: hsl(222, 99%, 18%);
} */

/* Select every 3th item in the list with an offset of two from the top. */
/* li:nth-child(3n + 2) {
  background: hsl(222, 99%, 80%);
  color: hsl(222, 99%, 18%);
} */

External CSS

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External JavaScript

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