To change the brightness via keypress i hacked some scripts together (some time ago) because it was easier (faster) to do that than to do it properly.
Here are the scripts: https://github.com/d13g4/brightnesshack
In the i3 config i had to add the following lines to the end:
bindcode 233 exec .config/bright_inc.sh bindcode 232 exec .config/bright_dec.sh
Your keycodes may be different, so make sure to get the right ones for you. These codes are the F5 (dec) and F6 (inc) keys on my (thinkpad a275 – german version) keyboard.