Word, being a first-party Microsoft program, is one of those few programs that actually abide by Windows accessibility features. As such, it uses system-level font/color settings.
You have to use a different Windows theme altogether (such as one of the “High-Contrast” ones) or manually edit the colors in:
Control Panel\All Control Panel Items\Personalization\Color and Appearance (Win 8, similar for 7, slightly different for XP)
This will affect some applications beyond Word, mind you, depending on how they were programmed.