Add cssclass dynamically in code behind using c#

Add cssclass dynamically in code behind using c#

Hi i want to add Cssclass dyamically in code behind.Its easy to add,But i
have little diffrent structure.
I have one master page which have one div like this:
<div class="contnav_bg" id="wrapper">
// some content
<div>
Now what want that class="contnav_bg" is applied only to dashboard.aspx
page.Right now it applies to all pages.
Can i have to create function for it or anything else.
Thanks