The old colors were Icon Grey (#4d4d4d) and an undocument white color (#f2f2f2). The new colors are Shade Black (#232629) and Cardboard Grey (#eff0f1).
With this change, icons in GTK applications will be the same color as icons in Qt applications with the Breeze and Breeze Dark colorschemes.
I used `sed --follow-symlinks -i 's/4d4d4d/232629/' *.svg` and `sed --follow-symlinks -i 's/f2f2f2/eff0f1/' *.svg` in each directory containing monochrome icons, excluding the color icons in `apps/16` and `apps/22`