Most desktop monitors have their brightness & other config exposed via the DDC (DDC?) protocol. This is a simple UI that exposes the brightness controls on Windows. Much easier than using the buttons.
You might need to go to the monitor's menu and enable DDC if it's disabled by default.
On Linux this script works for me:
sudo modprobe i2c-dev; sudo ddcutil setvcp 10 $1 &
On macOS I've been using https://github.com/the0neyouseek/MonitorControl -- which also lets you use the brightness keys to control external monitors.
Just realize when you use them as a justification for changing defaults which work fine for most everyone else, you're going to annoy people. Especially when your preferences tend to fly in the face of common sense.
Not everything has a dark mode available, so make sure that the worst case is tolerable.