Hello
Keyboard light is not a Windows feature. It is a feature from the hardware/firmware. So it is not possible to adjust it via Windows settings.
Whether the light can be turned up and down depends on the model itself. If it is supported, typically the same key that turns it on and off can allow you to turn it up or down with each press. If it is not there, then I am afraid it is not supported. Or you can contact Asus support to inquire if your specific model supports this feature.