This is a pretty simple edit. Required: ThemePark and a copy of the Extras.rsrc file located in System -> Library -> Frameworks -> Carbon.framework -> Versions -> A -> Frameworks -> HIToolbox.frameworks/Resources. Make a copy by option-dragging it to the desktop.
Open the copy of Extras.rsrc in Themepark and select clr# (384); edit the last color which is the original light blue. Click on it and change the color to any color you want. Then save and close the program.
Either login as root or reboot in OS9 and replace the old Extras.rsrc with the new edited Extras.rsrc. Restart or login. That is it. It is that simple.

