Ok for Windows its called Screen Tinter and its FREE from Here
For Apple Mac its called Nocturne and is available Here and is also free.
With the Mac version the settings i found that work really well are as per screen shot ( choosing your own preferred colour overlay that is! )
Also if you drag the Application to the Dock of your mac, when ( the application ) is set up with your preferences , simply click on the “alias” on the dock to either activate / deactivate it!
Works a treat 🙂