Quote: "It contains a shade more than that, and not exposed as resources either."
Erm, yes of course it does. But you asked why it's referenced to in .theme files.
AS for common controls:
Quote: "Using Common Controls
Most common controls belong to a window class defined in the common control DLL. The window class and the corresponding window procedure define the properties, appearance, and behavior of the control. To ensure that the common control DLL is loaded, include the InitCommonControlsEx function in your application. You create a common control by specifying the name of the window class when calling the CreateWindowEx function or by specifying the appropriate class name in a dialog box template.
DLL Versions
All 32-bit versions of Windows include a common controls DLL, Comctl32.dll. However, this DLL has been updated several times since it was first introduced. Each successive version supports the features and application programming interface (API) of earlier versions. However, each new version also contains a number of new features and a correspondingly larger API. Applications must be aware of which version of Comctl32.dll is installed on a system, and only use the features and API that the DLL supports.
Because new versions of the common controls were distributed with Microsoft Internet Explorer, the version of Commctl32.dll that is present is commonly different from the version that was shipped with the operating system. It may actually be several versions more recent. It is thus not enough for your application to know which operating system it is running on. It must directly determine which version of Comctl32.dll is present. For a detailed discussion of common controls versions and how to determine which version of Comctl32.dll is installed, see Shell and Common Controls Versions. "
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/wincontrols.asp
If you take your time and browse through the MSDN you'll also find more information about shell32.dll, the difference between comctl32.dll and shell32.dll, and what they're all for.
