To add a custom CSS file in the Style Library to a customer master in SharePoint 2010, add the following line on the Master Page just before the closing head tag: <SharePoint:CssRegistration name="<% $SPUrl:~SiteCollection/Style Library/YourStyleSheet.css %>" After="corev4.css" runat="server"/> This would load the custom style sheet after the SharePoint 2010 core style sheets.
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