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To invert the entire screen you can use the program xcalib:

xcalib -invert -alter

From the xcalib man page

xcalib loads 'vcgt'-tag of ICC profiles to the X-server using the XVidMode Extension in order to calibrate your display.

That means it does not depend on compiz being used.

You can install it using sudo apt-get install xcalib. To make it more convenient assign a keyboard shortcut opening the Keyboard settings.

There, navigate to shortcuts. Add a custom shortcut by pressing the plus button, add the name and the command. It will be disabled by default, so assign a shortcut by clicking on Disabled and perform the wanted shortcut, e.g. meta + i

Before:

After xcalib -invert -alter was launched (mapped to <meta-i>:

Pressing it again restores to the normal not inverted screen.

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