Hi,
Here's an image taken from the [https://support.casio.com/pdf/004/fx570MS_991MS_E.pdf|Additional functions User guide 2] for the calculator, that may help.
It doesn't specify exactly what setup options are available if you press the Mode button >3 times but it says that it actually changes the calculator setup so perhaps the brightness function is in there. Worth a look perhaps.
[image|2824671]
(click on image)