Posting this for future reference:
Dim header As Web.UI.HtmlControls.HtmlHead
header = TryCast(Me.Page.Header, Web.UI.HtmlControls.HtmlHead)
If (header IsNot Nothing) Then
Dim link As New HtmlLink
link.Attributes.Add("type", "text/css")
link.Attributes.Add("rel", "stylesheet")
link.Attributes.Add("media", "screen")
link.Attributes.Add("href", "~/style.css")
header.Controls.Add(link)
End If