Inexplicably, the Game Boy Advance has a hidden contrast control. Find out where inside ...
While there is no control on the unit for it, the Game Boy Advance apparently has a hidden contrast control, if you don't mind fiddling with potentiometers, or voiding your warranty.
Underneath the serial number label on the back of the GBA, on the far right hand side is a hole leading to the pot. Stick a screwdriver (gently!) through the hole, punctuing the label, and you should be able to adjust the contrast.
However, it is HIGHLY recommended that you leave it be. It doesn't make games any easier to see without lighting, and has the potential (no pun intended) of screwing up your GBA. Don't say we didn't warn you.
Thanks to IGNPocket for the revelation.