After upgrading my Toshiba Z830 Ultrabook to 12.04 (Precise Pangolin) I noticed that the ability to control the screen back-light was not working using the Toshiba Fn F6/F7 keys.

Thanks to http://www.linlap.com/wiki/acer+aspire+s3 the solution was found quite quickly.

sudo vim /etc/default/grub

This will open the grub configuration file. (Grub is the initial boot selection software)
To be able to dim the screen brightness, You’ve got to modify the line: