Display brightness
Use brightnessctl. Do not open a settings GUI for this.
brightnessctl get # current value
brightnessctl max # maximum
brightnessctl set 40% # absolute
brightnessctl set 10%+ # relative
brightnessctl set 10%-
Tier: reversible. Undo: brightnessctl set <previous> (read it first with get).
Verify
brightnessctl get prints the new value. On a VM or headless system there may be no backlight device; say so instead of trying GUI paths.