A lot of phones come with illuminated softkeys these days.
As they’re easy too use and visually pleasing, the light they emit is annoying in certain cases. Good examples are if you want to watch a movie at night or if you use your phone as a alarm clock (on a docking-station). Some manufacturers like Samsung provide options to disable the light under certain circumstances or completely, others like HTC do not give control about this.
If your phone is rooted, the solution to this problem lies in the Android configuration itself: The file /sys/class/leds/button-backlight/brightness contains the value for brightness in a range from 0 (light out) to 255 (full brightness). Simply open it with a text editor and change the value.
On some devices (e.g. devices with HTC Sense), the system resets the file if it finds any changes to it, so the changes will only be temporary. The solution for this is to change the permissions to the file to read-only (chmod 444) after editing it. If you want to change it back simply give back writing-permission (chmod 666).
As the whole procedure is a bit complicated and most users only want to disable the light under certain conditions, it would be very useful to have a Tasker task for doing that. Luckily there are people like jayelbird on xda-developers who just did this: Link
I personally use a task which disables the lights whenever I put my phone onto my dockingstation. In combination with Dock Clock I’m able to use my phone as a alarm clock. Before I was able to turn out the lights, the phone illuminated my bedroom the whole night.