n-o-d-e (blogger) has published many mods for this watch, with videos included. They have great production quality, and the mods are easy enough to pull off. https://n-o-d-e.net/casio.html

When I did this mod, I soldered the LED on its side, so that it shone directly onto the LCD. This made the watch face much brighter.

On reflection, the similar variant of the watch with an EL display is vastly superior. The A168 is similar to the A158 you mentioned but with a better light.