change theme of page dynamically

protected void Page_Load(object sender, EventArgs e)
{    Page.Header.Controls.Add( gettheme());

}

protected HtmlLink gettheme()
{
HtmlLink lk = new HtmlLink();
// <link type="text/css" href=StyleSheet.css rel=Stylesheet />
lk.Attributes.Add("type", "text/css");
lk.Attributes.Add("href", "StyleSheet1.css");
lk.Attributes.Add("rel", "Stylesheet");
return lk;
}