Add Header link in C# Code Behind and Front

use HtmlHead and HtmlLink for this job

HtmlHead header = (HtmlHead) Page.Header;
HtmlLink link = new HtmlLink();
link.Attributes.Add( "href", Page.ResolveClientUrl( "~/App_Themes/Default/Default.css" ) );
link.Attributes.Add( "type", "text/css");
link.Attributes.Add( "rel", "stylesheet");
header.Controls.Add(link);