Problem
One can use xrandr to adjust screen brightness. However, the factor (default to 1.0) is relative to the current brightness. How to enable the brightness adjust keys / on the keyboard under Xubuntu 18.04?
Solution
Toz’s post on Ubuntu Forum works like a charm. The default value
for GRUB_CMDLINE_LINUX is an empty string. Changing it to
acpi_backlight=vendor and updating the GRUB has solved the
problem since the next boot.