I no longer remember off the top of my head, and my secondary N900 (on which I don't mind installing and uninstalling stuff just to check that) is currently incapacitated, but I took the time to search maemo.org/packages/, download the FileBox .deb, extract data.tar from it, and look at the file path of the FileBox themes, so the answer is:

/opt/filebox/icons/

is where the FileBox theme files reside. So if you want to just drop the BlackHumanity folder in, you'd just copy it to that location (so you'd have /opt/filebox/icons/BlackHumanity/*

If you wanted to insert the individual icons into another FileBox theme, you'd put them into their respective locations in one of the other folders already in /opt/filebox/icons/