mooq,

That color is hard coded on XP and the only way to change it is adding
custom .UIFile to the skin, they are custom XML-like files that control
the task panels but they are a bit complicated to deal since you have to
manually edit the code by hand.

This skin use these custom files: https://www.wincustomize.com/skins.aspx?skinid=5705&libid=1

inside the skin folder you'll se file with the extension .UIFile open 'em with
Notepad or other text editor to check.

try google for "UIFile tutorial" too, I remember there's one somewhere.