Function keys are not working on Toshiba Satellite L510 so brightness control is not possible

Open a terminal using Ctrl+Alt+t and type sudo apt-get install xbacklight. That will install xbacklight if it isn't already there (I can't remember whether it's a default package!).

Then type xbacklight -set x replacing the x with a number between 1 and 99 to set the percentage brightness for your screen backlight. You can actually set to 0 (and I do because I hate bright screens) but I understand that can cause problems on some displays so I don't recommend trying it.

I just wanted to add, you can then set a keyboard shortcut to something like xbacklight -inc 10 to increase the brightness by 10%.

Unfortunately, you can't use Fn + Another Key since like he said, Fn is reserved for the BIOS. It's weird because my Fn + F7 works in turning off/on the screen, and Fn + F2 works in turning off (but not on) my wifi, but F5 and F6 do not adjust brightness... Hope someone can find a real fix soon.