leave the light as as-is and sell to a collector. don't make rare shit more rare when you could backlight a pocket instead and come out ahead money-wise

Well Jeff, seems you are right and I have an issue with my GB Light
This 1st picture is the original backlight and is so dim

Then I tested with a pocket screen with a backlight and is a huge difference, actually the original seems brighter than it is

And later I tried the original backlight with the polarizer film of the green backlight and yes is the same brightness

So my issue seems to be the screen polarizer of my GB Light, and that I found in internet but didn't thought that was my case
https://imgur.com/a/K6pFH
Now the thing is that with the original backlight the background looks dirty, and with the backlight kit not.
Thinking how should I go on

The back-light is pretty decent on the Game Boy light. I don't think it would be any easier than back-lighting a Game Boy Pocket but with relatively little to gain.

I searched but could not find any information regarding the GameBoy Light, yes it has a backlight but is not as good as modern backlights.
As GameBoy Light uses 2xAA instead of 2xAAA should not be an issue using everdrive and batteries can last longer.
And as it has a switch to turn it off/on/backlight could be helpfull.