Currently there are no CSS variations of a skin the way you describe.
So you would have to use some custom code to do this (which will only work if you hardcode it to the portal in the skin, I think)
If you add this to your skin.ascx file:
<script runat="server">
Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
SetCSS()
End Sub
Private Sub AddCssLink(ByVal sName As String)
Dim oLink As New HtmlGenericControl("link")
oLink.Attributes("rel") = "stylesheet"
oLink.Attributes("type") = "text/css"
oLink.Attributes("href") = String.Concat(SkinPath, sName)
Dim oCSS As Control = Me.Page.FindControl("CSS")
If Not oCSS Is Nothing Then
oCSS.Controls.AddAt(0, oLink)
End If
End Sub
Private Sub SetCSS()
AddCssLink(String.Concat("skin", PortalSettings.PortalId, ".css"))
End Sub
</script>
It will load an extra CSS file based on the portal id.
(I would not replace the skin.css file with skin1.css etc, but leave all that they share in skin.css and put the rest in skinx.css)
You could also use a Javascript solution or you could use Portalsettings.PortalName instead of the portal id
btw, you can also use includes in the ascx, put everything in the ascx in "myinclude.inc" and create different ascx files with this include...
(so a change in the include wil be visible in all the skins)