Some time developers need to call the css from code behind or dynamically from content page then you may call css with below given code.

HtmlLink cssLink = new HtmlLink();
cssLink.Attributes.Add("type", "text/css");
cssLink.Attributes.Add("rel", "stylesheet");
cssLink.Attributes.Add("href", "~/MyStyleSheet.css");
this.Header.Controls.Add(cssLink);