Add this to you skin (on top below the SKO registrations):

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        Dim sSkin As String = PortalSettings.ActiveTab.FullUrl.Replace("http://", "")
        sSkin = sSkin.Split(".")(0)
        sSkin = sSkin.Split("/")(0)
        sSkin = sSkin & ".css"

        Dim oLink As New HtmlGenericControl

        oLink = New HtmlGenericControl("link")

        oLink.Attributes("href") = PortalSettings.ActiveTab.SkinPath & sSkin
        oLink.Attributes("type") = "text/css"
        oLink.Attributes("rel") = "stylesheet"

        If Not oLink Is Nothing Then
            Dim oCSS As Control = Me.Page.FindControl("CSS")

            Dim iLocation As Integer = 0
            iLocation = oCSS.Controls.Count - 1

            If Not oCSS Is Nothing Then
                oCSS.Controls.AddAt(iLocation, oLink)
            End If
        End If


    End Sub