Tested with Intel GPU (630) using modesetting driver and acpilight package
Steps to enable backlight keys using acpilight and XF86MonBrightness{Up|Down} on Openbox based systems.
Install the package acpilight.
Add yourself to the group video
sudo gpasswd -a $USER video
Restart your system - otherwise acpilight may not function as expected.
The following should not be necessary - provided as an alternative
Ensure your user is member of wheel group (the install user on Manjaro is member of the group)
Add the following rule to /etc/sudoers.d to allow xbacklight to be run without password
echo "%wheel ALL=(ALL) NOPASSWD:/usr/bin/xbacklight" | sudo tee -i /etc/sudoers.d/10-xbacklight
Then create an alias in .zshrc (openbox has used zsh for years) so unless you changed it this should do
echo 'alias xbacklight="sudo xbacklight"' >> ~/.zshrc
Log off and login to make sure it work and the alias is loaded. Now you can set backlight using the designated keys as defined in ~/.config/openbox/rc.xml.