First hold the ESC button with computer turned off, then turn it on and keep the button pressed until "Check system." message appears. Then press F1 (this is how it works on my L70).
The screen brightness can be adjusted by software utility in Windows 95, I don't know about other methods. But after powering on the brightness is probably already maximum and mine is also not too bright. The solution probably would be replacing CCFL lighting tube with a new one, because brightness is getting worse with age. If my backlight becomes broken I will try replacing it with LED light strip 😜