UI: Settings' scrollbar not working on Firefox

[royalmo]

In Firefox, the scrollbar isn't working:

@FlyingKoala01 maybe you want to take a look, but don't worry, it's not for right now :)

[royalmo]

Here are some links with the info you need:

https://stackoverflow.com/questions/6165472/custom-css-scrollbar-for-firefox/54101063#54101063
https://stackoverflow.com/questions/6165472/custom-css-scrollbar-for-firefox/53739309#53739309

In fact, all the answers have important information, so why not reading them all?

[royalmo]

Now trello found something that could be useful. For future reference:

scrollbar-color: var(--ds-background-neutral-hovered,#091e4224) var(--ds-background-neutral,#091e420f);
scrollbar-width: thin;