Malcolm,

I am just about to do this for a .NET control but I believe you need to do some of the following:-

1] Determine if the version of the OS supports themes.
2] If it does find out if themes are in use using something like:-

declare integer IsThemeActive in UxTheme

3] Now pick the relevant colour indexes for the gradient using the following:-

declare integer GetSysColor in user32 integer nIndex 

I think the gradient is based on a combination of the following entries:-

COLOR_GRADIENTACTIVECAPTION
COLOR_ACTIVECAPTION

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/sysinfo/base/getsyscolor.asp

Sorry for being vague, but like I said I am about to start implementing this support.

Regards
Neil