It has been quite popular to do backlight and bivert mod for GameBoy Pocket. I am going to try that myself as well.
There aren't much worth talking, just remove the reflector in the screen and install the backlight. I didn't have a good backlight module, so I decided to trim the backlight I got from a 3.2" TFT LCD and fit there.
To talk about the polarizer or bivert mod, I need to go into the STN/FSTN display mode. If one have tried to buy some 12864 or 1602 LCD modules online, one would know they came in yellow-green and blue-purple these 2 colors. It depends on the how the polarizers are installed on the screen. In positive mode, the front and back polarizer are parallel. Undriven pixels allows light to pass through, and driven pixels block light. In negative mode, the polarizers are perpendicular. Undriven pixels block light. There are also the difference between STN and FSTN, the F refers film-compensation. The natural color of STN is green (positive) / blue (negative), in FSTN that's compensated to white and black by the film. As a summary, there are 4 possible combinations:
No compensation (STN), parallel polarizing (positive): Green background, blue text No compensation (STN), perpendicular polarizing (negative): Purple background, white text Film compensation (FSTN), parallel polarizing (positive): Green background, black text No compensation (STN), perpendicular polarizing (negative): Green background, blue text
It's also quite simple, remove the old polarizer and install the new one. Note that the GBP uses FSTN, so it has the compensation film. To mod into blue/purple background, one would need to remove the compensation film as well. Note that as the color is inverted, the data should be inverted as well.
So here is result. The left hand side is the more popular mod, mod into STN negative mode. Personally I prefer modding into FSTN negative mode, which is on the right.