I had some free time, so I managed to do it myself (not manually):

But before we begin, since I don't know where XP stores its classic color schemes, we are going to bypass the problem entirely.

You will need to use the "Classic theme configurator" tool from here, which I'll refer to as ThemeTool from now on.
Once ThemeTool is on your machine, execute it now just to see if everything's working. Remember that different EXE's are required for different OSes.
Download any one of these files (rename them to .reg), keeping in mind the following:
- Anything that says (ThemeTool) requires ThemeTool. If it doesn't, it requires Windows 9x. Keep in mind that the ThemeTool versions are usable on all OSes in which Classic Theme is usable, but the 9x versions are 9x-specific, obviously. The ThemeTool versions also include Windows Standard, so you can revert back to default if necessary.
- Anything that says (Tweak) just means I change the Tooltip background to the 3D Object color, and the text to the main text color. It looks like an honest mistake that I think anyone could have made, so I fixed it. However, I provide the original versions for posterity.

Windows XP Classic Theme Support #5

Description