My C5 used to have 3 dash light settings, I think accessed by a press of right hand button in instrument pod, and also by pressing the half white/black button near the stereo.
setting :
1 # all dash lights on,
2 # just dials back-light
3 # just the speedo
3# is no longer accessible ?
Its quite handy for making people go quiet in the car, like turning the lights off
I do have a handbook but cant put hand on it right now..
John