OK, I've seen some posts about this... and a few minutes ago I noticed that my MBA's keyboard was not lighting up as it's supposed to. A little experimenting and I found the solution (you may kick yourself, this is so simple):
The F5 key decreases keyboard brightness.
The F6 key increases keyboard brightness.
The icons on these two keys are the clue.
The F5 key decreases keyboard brightness.
The F6 key increases keyboard brightness.
The icons on these two keys are the clue.