<ol>
  <li>ここは1番目の項目</li>
  <li>ここは2番目の項目</li>
  <li>ここは3番目の項目</li>
  <li>ここは4番目の項目</li>
  <li>ここは5番目の項目</li>
</ol>
li:nth-child(odd) {
  color:red;
}
li:nth-child(even) {
  color:blue;
}

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