What brightness does the brt and dim on the right side of the MCDU MENU button change?
The brightness of the display, the keys or of both?
And if only one of them, what controls the other?

(source: skalarki-electronics.eu)
EDIT: As shown in this video, the brigghtness knob turns just the display on/regulates it's brightness. What changes the brightness of the keys (They seem to be lit befor the MCDU is turned on)?