This is a gnome-shell extension to dim the multimonitor desktop with one click.

This extension adds a button with a light bulb to your panel. When clicked, the desktop brightness is reduced. Scroll to change the brightness. The main purpose is to keep your eyes calm when reading in the late evening. It is realized by a simple overlay with 70% opacity. It should work on a multimonitor desktop.

The extension can be installed from here: https://extensions.gnome.org/extension/1129/dim-desktop-70/ The number in the name comes from the default 70% opacity which was the only possible value in the initial release.