Why Style of DropDownList is automatically changing when clicking button in an UpdatePanel?

Hi,
   
         I am doing an ASP.Net web application with Framework 3.5.In my application am using UpdatePanel for avoiding page refreshing while changing SelectedIndex of DropDownList.The problem is that while clicking the button in the UpdatePanel the style of the DropDownList is changing(I am using template and CSS for style).Please help me to rectify these problem.

Best regards,
Fuad Pulpadan