Option to change opacity on chat boxes

Suggestions
1

Imported from legacy Flarum.
Original author: KaptainPickles (Flarum #722)

Right now it can be difficult to read some of the chat depending on what part of the map it is covering. A darker background/map area makes things a bit more legible. It would be really nice to see a feature that lets you modify the opacity for those of us that have crappy eyes.

2

Imported from legacy Flarum.
Original author: jinbou (Flarum #618)

I remember there was a command for this in one of the old servers I played, we could write something like @opacity followed by a number to adjust chatbox opacity. was useful.