It doesn’t support keyboard shortcuts or user-defined presets. The only downside is a lack of controls within the app. So, you just have to extract the zip folder and double-click to use Dimmer. I tried it on 3 monitor setup and it worked flawlessly.
The app casts a grey overlay on all the displays and you can just move the slider to dim the screens. Dimmer, as the name suggests, is a basic Windows screen dimming app that supports multiple displays.
