subbziro,
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.mainlast_li {
background-color: #1b6d85;
}
</style>
<script src="//code.jquery.com/jquery-1.11.3.min.js"></script>
</head>
<body>
<ul class="main_ul ">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
</ul>
<script>
if(!$('.main_ul li').length % 2 == 0){
$('.main_ul li:last-child').addClass('mainlast_li');
}
</script>
</body>
</html>