Jeremiah Morris - 2014-02-23

Unfortunately, the brightness setting relies on features in OS X that stopped working properly in Mavericks. On some machines, the older Aleph One will crash, or screw up the colors. We disabled brightness under 10.9 to protect the unlucky ones.

Brightness continues to work on older versions of OS X, or under the Software renderer where it's implemented differently. We do plan to restore the setting for everyone in a future release, but it will take time to put together a solution that does not rely on the now-broken OS routines.

In the meantime, you can create a new display profile in Displays preferences (click Calibrate... in the Color tab) with a different Target Gamma (turn on Expert Mode for a slider), and switch to that profile when you play. Setting the brightness there will affect Aleph One's game window.