How to add link to a css file in content page

Try this

In VB

Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init
   Dim css As HtmlGenericControl
   css = New HtmlGenericControl
   css.TagName = "style"
   css.Attributes.Add("type", "text/css")
   css.InnerHtml = "@import ""/foobar.css"";"
   Page.Header.Controls.Add(css)
End Sub

In C#

protected void Page_Init(object sender, System.EventArgs e)
{
   HtmlGenericControl css;
   css = new HtmlGenericControl();
   css.TagName = "style";
   css.Attributes.Add("type", "text/css");
   css.InnerHtml = "@import \"/foobar.css\";";
   Page.Header.Controls.Add(css);
}