Using CSS and Changing them Dynamically ASP.NET & C#

Found this Here:-
http://www.aspnettutorials.com/tutorials/themes/usingcss-csharp.aspz

basically:-

add 'id' to this tag
<link href="dark.css" rel="stylesheet" type="text/css" id="stylesheet" />

and in code behind:

stylesheet.Href = "dark.css";