How to add a property in existing css class using javascript?

Question

I want to add a css property in a class using JavaScript.

Example

<style type="text/css">
.AClass{
   color:#ccc;
   padding:5px;   
}
</style>

I want add a property height:500px; in this class using javascript, not on that element which class is AClass. So finally my class should be

 <style type="text/css">
    .AClass{
       color:#ccc;
       padding:5px;   
       height:500px;
    }
    </style>

But programmatically.

Please Advice me