sudo apt-get install xbacklight
it will install the program, which worked great on my LXDE install on Toshiba Chromebook. xbacklight will give you the % of screen brightness, you can see the list of commands, but xbacklight -dec XX will decrease BY XX percentage, -inc XX increase.
So the last operating system I discussed in any depth was Fedora 20, which I installed around October 2014. Given the length of support that Red Hat gives any edition of Fedora, I was lucky that it was maintained until June 2015 owing to delays to Fedora 21. 470 more words