HtmlLink cssLink = new HtmlLink();
cssLink.Href = "%CSS FILE URL HERE%";
cssLink.Attributes.Add( "rel", "stylesheet" );
cssLink.Attributes.Add("type", "text/css");
Master.Page.Header.Controls.Add(cssLink);

work from content pages or submaster pages

adding CSS <link> to a C# Master page from a child code-behind page

css, link, page

C#