Андрей Федоров

.main li:not(:last-child) a:after{
    position: relative;
    color: #fff;
    content: " | ";
    display: block;
}

div {
  height: 100vh;
  width: 100vw;
}

<div>
  <div>Нечетный</div>
  <div>Четный</div>
</div>

div div {
  width: 200px;
  height: 200px;
}
div div:nth-child(even) {
  background-color: red;
}
div div:nth-child(odd) {
  background-color: green;
}