Themes in SharePoint Server 2010 are a feature of the core product and work by default with both the out-of-the-box v4.master and nightandday.master pages, unlike SharePoint Server 2007 where, by default, the custom (publishing) master pages, such as BlackBand.master, ignored a site’s theme.
In SharePoint 2007 the default.master page, which is the equivalent of the v4.master page in SharePoint 2010, included the
<SharePoint:Theme runat="server"/> control and
<meta name="Microsoft Theme" content="Themename 1011, default"> tag, which instructed SharePoint that a theme has been applied to the site and to reference customized theme files after the non-themed CSS files. In SharePoint 2010, ...