sinergia | ️ Cooperative expensive tasks at 60fps via ES6 generators | Machine Learning library

I'd like to have the 9 button (3x3) to get a color change after getting clicked, so i used a .btn:focus class where I set a bg-color and a text color. the problem is the fact that if I click somewhere else (in the blank spot of the page or in the radio type input below) this color change disappears, but I need it to be there. It should disappear only if I click a different button of the same group of 9 but I don't know how to do it.

I tried with a JS function in the lower part of the HTML code that should add to my 9 buttons a class that should color them but that doesn't work. (I've just realised that even if this last JS function would work, my problem would be the fact that if I click a button that is not the one already selected I'd have 2 different button colored. I don't know how to solve my problem.)