Just remember there are thousands of non-microsoft visual styles. Thought Windows XP allows only standard visual styles many people use patched UxTheme.dll or an application like StyleXP to allow custom visual styles. AFAIK Windows XP Media Center Edition also has visual styles different from what we found in Home or Professional Edition.
And I believe that API indicates that theme is active for custom styles too.

Yes, you're right.
Actually I have predefined styles for Blue, Silver and Olive.
Blue theme is "default" theme when you're not using Silver and Olive themes.
Is exactly this that I would like to change.