Add or override the brightness slider to change the brightness via an alpha layer (and optionally stop using or cooperate with the exising backlight, if present). Either internal, external or all monitors can be dimmed. See the GitHub page for details. Note that this extension will keep running on the lock screen, as you'd also want the brightness setting to apply to the lock screen as well. Please report on GitHub if this gives you any trouble.
Note: Binary files aren't shown on the web site. To see all files, please download the extension zipfile.