You cannot edit the .msstyles if it is in use switch to the default Aero theme log off and then log on and then open up Windows style builder.
You will need to navigate to the theme .msstyles to edit it. Look at screenshots below.
I do not have Microsoft Word so little be hard for me to test unless you can come up with another program(That is free or has a trial) or area that you have the same problem with and let me know.
You could try changing the color at
Fonts, Colors & System Metrics > Fonts & System Metrics > WINDOWS: COLOR
and then use the test button and see if that get it if not let me know.