This is a very dirty fix! It will change the opacity of all owned games anywhere in the store but does not take settings made in Barefoot Essentials into account. It's probably not performant at all and functionality was only tested on Firefox 63.0.1 using Violentmonkey 2.9.9.

Your browser needs to support the DOMSubtreeModified event. You can check wether your browser supports it here:
https://caniuse.com/#search=DOMSubtreeModified