All controls prefixed with SharePoint or WebPartPages are part of the SharePoint framework and can be found on the Web server in the assembly, Microsoft.SharePoint.dll. All SharePoint pages contain SharePoint Server controls, some of which are shown in the Toolbox task pane as follows:
CssLink. Links to CSS files. By default this control is placed on master pages and links to a file stored on each Web server, so all pages associated with a master page have the same look and feel. When the control is used with no file specified, it defaults to core.css.
Theme. Adds a reference to the site’s current theme CSS file, if one is configured.