You could maybe try the Linux command "xgamma -gamma x.x" where "x.x" is the number of your desired gamma, with 1.0 being the default value. "xgamma -gamma 1.5" would then be brighter, while "xgamma -gamma 0.5" would be darker. Try running that on a terminal before running the game or using alt-tab once the game is running.
Just a thought
Good luck!