Your answer would depend on where the CSS lives.  If it is in the skin package, you might do it like this:

Public ReadOnly Property CssPath() As String
    Get
        Dim p_settings As New PortalSettings
        p_settings = DotNetNuke.Entities.Portals.PortalController.GetCurrentPortalSettings
        Return String.Concat(p_settings.ActiveTab.SkinPath, "control_style.css")
    End Get
End Property

<link rel="stylesheet" type="text/css" href="<%=CssPath%>" />