You don’t often see a brightness control on a website but if you ever need to show one this UI designed by Mariusz Dabrowski could just what you need. When you click on the button in the bottom right it reveals as range slider that you can interact with to increase or decrease the brightness of the background.
If you are having trouble with the pen, try the archived copy on GitHub
Brightness Control Snippet
See the Pen Brightness Toggle by Mariusz Dabrowski (@MarioD) on CodePen.0