Please be more precise about your hardware (what video card?, does the laptop
have function keys to adjust the brightness?) and about what you have
attempted so far (I guess you tried to add "acpi_backlight=vendor" to
GRUB_CMDLINE_LINUX_DEFAULT and "acpi_osi=Linux" to GRUB_CMDLINE_LINUX but
that is only a guess).
Assuming you use an Intel graphical chipset ('lspci', in a terminal, would
tell), does the following command (where 60 is an example of brightness
value) changes the brightness?
$ echo 60 | sudo tee /sys/class/backlight/intel_backlight/brightness
If you use an nVidia card, the file should be
/sys/class/backlight/nv_backlight/brightness instead of
/sys/class/backlight/intel_backlight/brightness
If it does work, your preferred value can be set at startup by writing
something like that (change the value and maybe the file) before the exit
command in /etc/rc.local:
echo 60 > /sys/class/backlight/intel_backlight/brightness
A more recent kernel may help too (including to get function keys properly
working): https://jxself.org/linux-libre/
ADFENO also wrote a script to easily adjust the brightness when the function
keys do not work:
https://trisquel.info/forum/grub-bootloader-trick-doesnt-work#comment-112392
