Why won't you just add something like this to MasterPage markup?
<link type="text/css" href="<%=Page.ResolveClientUrl("~/css/Styles.css") %>" rel="Stylesheet" />
If you want to control css files from aspx Pages just add content Control:
<head runat="server">
<asp:ContentPlaceHolder runat="server" ID="pageHeadContent">
</asp:ContentPlaceHolder>
</head>
and control this content place holder from page like this:
<asp:Content ID="cwbHeadContent" ContentPlaceHolderID="pageHeadContent" runat="server">
<link type="text/css" href="<%=Page.ResolveClientUrl("~/css/cwb.css") %>" rel="stylesheet" />
</asp:Content>
But remember to NOT place anything in that placeholder in MasterPage AND Page, cause content in MasterPage just disapper.