focus ring

Hello,
my name is Isabel. I need some help to make same changes in Mozilla.

I would like to make some changes in the mozilla's focus. I know how
to do same of this changes(backround color, ring width...) by changing
the file:
modules/libpref/src/init/all
.js

However, I don't know where is implemented its functionality. For
example, I want to change the color of the element focused's ring, but
I don=B4t know where is that function. I mean, where is said that the
ring is dotted and black.

Sorry about my english. Thank you very much.

Isabel.