Add your ContentPlaceHolder to your tag head:
<head>
<asp:ContentPlaceHolder ID="head" runat="server"></asp:ContentPlaceHolder>
</head>
Then on your content page:
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<link href="Skins/SkinCustom/Editor.Default.css" rel="stylesheet" type="text/css" />
</asp:Content>
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