Для вставки ссылок на CSS файл вы можете использовать следующие подходы
<SharePoint:CssLink runat="server" Version="4"/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<SharePoint:Theme runat="server"/>
Создает код
<link rel=”stylesheet” type=”text/css” href=”/_themes/[UniqueCounter#]/corev4-[u=UniqueHex#].css?ctag=[UniqueCounter#]”/>
<SharePoint:CssRegistration name=”customfolder/samplecustom.css” runat=”server”/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/customfolder/samplecustom.css"/>
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<SharePoint:CssRegistration name=”customfolder/samplecustom.css” After=”corev4.css” runat=”server”/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/customfolder/samplecustom.css"/>
<SharePoint:CssRegistration name="<% $SPUrl:~sitecollection/Style Library/samplecustom.css %>" After="corev4.css" runat="server"/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<link rel="stylesheet" type="text/css" href="/Style%20Library/emc_2010_custom.css"/>
<SharePoint:CssRegistration name="<% $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/samplecustom.css %>" After="corev4.css" runat="server"/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<link rel="stylesheet" type="text/css" href="/[sitename]/Style%20Library/en-US/Themable/Core%20Styles/emc_2010_custom.css"/>
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/customfolder/samplecustom.css"/>
Создает код
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/Themable/corev4.css"/>
<link rel="stylesheet" type="text/css" href="/_layouts/1033/styles/customfolder/samplecustom.css" />