Customizing your IntelliJ project appearance by setting a background image

When developing I sometimes have closely related projects open simultaneously.
And as you may expect, I end up messing around in the wrong project.
Here's a little trick to easily distinguish between projects: give the editors a different background image.
It's an option that is not even in the menu's, but you can search for the action to execute it, as explained in the manual. Make sure you check This project only.