If you use an ATI Radeon video card, and find that your screen looks a bit too bright in BeOS, you aren’t alone. Fortunately, this problem is easily fixed:
First, if you haven’t already, rename /boot/home/config/boot/UserSetupEnvironment.sample to UserSetupEnvironment. Then add the following to said file: export APP_GAMMA=1.0
Save and reboot. If you wish, you can tweak the gamma value from there. The smaller the value, the darker the screen.