How do I keep a li highlighted in JavaScript?

I know there are other topics on this subject, but none of them are noob friendly ;) I need to make a website for school, and I want to keep a li highlighted after I click it.

Here is what I've got:

<!DOCTYPE html>        
<html>
<title> De Vakman Wiersema </title>
<head>


<script type="text/javascript">
  <!--

  //-->
</script>
<style>

  body {
  background-color: #F2EEE6;
  } 
  a {
  text-decoration:none;
  color: #FFFFFF;
  }
  div.head {
  width: 1100px;
  height: 50px;
  margin-left: auto;
  margin-right: auto;
  margin-top: 20px;
  border: 1px solid black;
  background-color: #F2EEE0;      
  }
  div.underhead {
  width: 110px;
  height: 500px;
  border: 1px solid black;
  background-color: #F2EEE0;
  }
  div.background {
  background-color: #7F7A76;
  position: absolute;
  width: 100%;
  height: 50%;
  top: 0px;
  right: 0px;
  left: 0px; 
  }
  #img1 { 
  margin-left: 292px;
  margin-top: 10px;
  border: 1px solid black;
  } 
  #cssmenu {
  width: 1000px;
  font-family: Helvetica;
  color: #FFFFFF;
  position: absolute; 
  }
  #cssmenu ul {
  list-style-type:none;
  padding:10px;
  overflow:hidden;
  background-color:#98BF21;
  margin-left: 292px;
  border-radius: 4px 100px 4px 4px;  
  }
  #cssmenu a,
  #cssmenu a:link,{
  display:block;
  width:200px;
  height: 50px;
  border: 1px solid black;
  background-color:#98BF21;   
  }
  #cssmenu li { 
  float:left;
  margin-left: 50px;
  border-radius: 5px; 
  padding: 0px;
  display: inline-block;
  } 
  #cssmenu li a {
  background: #98BF21;
  display: block;
  padding: 10px;
  border-top-left-radius: 5px;
  border-top-right-radius: 5px;
  border-bottom-right-radius: 5px;
  border-bottom-left-radius: 5px;
  }     
  #cssmenu li a:hover {
  background: #11A304;
  }
  div.page {
  width: 990px;
  height: 800px;
  margin-left: auto;
  margin-right: auto;
  border: 1px solid black;
  background: #F7F5F5;
  border-radius: 5px;
  box-shadow: 0px 0px 20px 3px #7F7979;
  padding: 5px; 
  margin-top: 80px;    
  }

  </style>


  </head>

   <body> 
   <div class="background">
   <div class="logo">
    <a href="http://goo.gl/maps/jScbt" target="_blank">
      <img id="img1" src="images/logo.jpg" alt="Logo" height="150" width="220">
    </a>
   </div>
   <div id='cssmenu'>
    <ul>
      <li><a href="#home">Home</a></li>
      <li><a href="#producten">Producten</a></li>
      <li><a href="#klussendienst">Klussendienst</a></li>
    </ul>
  </div>
  <div class="page">
    Text
  </div>
  </div>
  </body>
  </html>

(sorry for not using any of the "jsfiddle" or any of the other programs:-(

(P.S. Terms on the website are in Dutch)

Solution

Without jQuery:

document.getElementById('cssmenu').onclick = function(e) {
    e = e || window.event;
    var t = e.srcElement || e.target;
    while(t != this && t.nodeName != "LI") t = t.parentNode;
    if( t.nodeName == "LI") {
        t.style.backgroundColor = "#11A304";
    }
};