Here’s a code snippet used to programmatically insert a stylesheet link to an external CSS file:
// Create the <link> element for the CSS file
var stylesheet = new HtmlLink { Href = "/path/to/stylesheet.css" };
stylesheet.Attributes.Add("rel","stylesheet");
stylesheet.Attributes.Add("type","text/css");
// Add it to the <head> element of the page
Page.Header.Controls.Add(stylesheet);