This particular I2C LCD does not have brightness adjustment capability. While it’s possible to use software simulated PWM, I suspect it’s going to be quite slow and may cause flickering. By the way you can email [email protected] and ask for a replacement green-colored LCD. We’ve recently started using green-colored LCD and the characters are visible even when the backlight is off. So this would work a lot better than the blue-colored LCD without having to upgrade firmware.