The following C# code adds a CSS link to the header node of an ASP.NET WebForm page.
// register css
HtmlHead head = (HtmlHead)Page.Header;
HtmlLink link = new HtmlLink();
link.Attributes.Add("href", Page.ResolveClientUrl("~/App_Themes/Default/StyleSheet.css"));
link.Attributes.Add("type", "text/css");
link.Attributes.Add("rel", "stylesheet");
head.Controls.Add(link);