#Bash - 57 <s>26</s>

    C=/s*/*/*/*/b*ess;(A=$(cat $C);echo 0;cat;echo $A)|tee $C

On a laptop this will set the screen backlight brightness to 0 via `/sys/class/backlight`, on a tablet or phone this will set the screen led brightness to 0 via `/sys/class/leds`