It's normally controled by a specific bitmap font, known as text.bmp.
Skinner atlas reference here.
However, by default in wa5, it switches off use of the skinned font, in favour of a default one from the local machine, to help with internationalisation, to switch it back on again, do..
CTRL+P > Preferences > Classic Skins > Check the "use skinned font for main window title display"
I'm not 100% sure where it gets it's colours from when it dosn't use the skinned font, my guess would be it's picking twp pixel areas from text.bmp and using that for the foreground/background colours.
Classic skins also look plug ugly when not using the skinned bitmap font.