Screen backlight controls work on 20.04 Live CD, but not once Ubuntu is installed

While installing Ubuntu 20.04 on my MacBook Pro 5,4 I saw that the screen brightness control keys on the keyboard did work to control screen brightness. However, once installed these keys now do nothing. The backlight control keys also work on the Ubuntu MATE 18.04.03 Live CD.

So, what is the difference between video driver or X setup between the Live CD and the installed system?

Thanks for any help.