HazeOver is an app (and website) designed to help you stay focused and productive while working on your computer. It automatically dims the windows and applications around your active window, drawing your attention to the task at hand and reducing distractions. You can customize the intensity of the dimming and define which applications you want to be dimmed. It also includes a “Do Not Disturb” mode, which will help you stay focused by preventing notifications and sounds from other applications. HazeOver also includes a timer that helps you to stick to the schedule and focus on the task without interruption.
The discontinued official website cannot be accessed, but the download is still available on the download sites.
Cinemadrape
Window Focus
Hocus Focus
WindowFX
UnDistracted for macOS
LeDimmer
Turn Off the Lights for Desktop
StayFocused
Clutter Cloak
Backdrop
Focus
It is exactly the same function, but 1) it works better and 2) it is cheaper. The other suggested apps help you concentrate on a task, they are not background window dimmers.