I am using Linux Mint 18. I have used this advice to install a custom play/pause key for Nightingale. I was pleased to find it worked, but then after minimising Nightingale, or bringing other programs on top of it on screen, the custom shortcut no longer worked. This is not ideal, and I am hoping someone can recommend an easy fix so that the shortcut works the play pause, no matter what position Nightingale is in. 
Custom shortcuts not working when program minimised
linux mint;keyboard shortcuts;cinnamon