Here are some of the most common ways to reference your custom CSS for SharePoint 2010. One key change over 2007 is the ability to specify After=”corev4.css” in the CssRegistration to make sure your custom CSS is referenced after the OOTB corev4.css file.

<SharePoint:CssLink runat=”server” Version=”4″/>
Note: Default OOTB
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>

<SharePoint:Theme runat=”server”/>
Note: If a theme is enabled
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_themes/[UniqueCounter#]/corev4-[u=UniqueHex#].css?ctag=[UniqueCounter#]”/>

<SharePoint:CssRegistration name=”customfolder/samplecustom.css” runat=”server”/>
Note: Alternate CSS on Server but comes before corev4.css
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/customfolder/samplecustom.css”/>
2. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>

<SharePoint:CssRegistration name=”customfolder/samplecustom.css” After=”corev4.css” runat=”server”/>
Note: Alternate CSS on Server and comes after corev4.css.
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>
2. <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”/>
Note: Alternate CSS In SharePoint Database for sites without publishing enabled
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>
2. <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”/>
Note: Alternate CSS In SharePoint Database for sites with publishing enabled
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>
2. <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”/>
Note: Alternate CSS on Server by using the standard link rel. Does not need the After attribute…
Output Order:

1. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/Themable/corev4.css”/>
2. <link rel=”stylesheet” type=”text/css” href=”/_layouts/1033/styles/customfolder/samplecustom.css” />