How to change background?

Hi!

First of all, greaaaaat plugin. I have used a few chat plugins and yours is the most user friendly by far.

I am trying to change the background of the chat (I am using the modern version) when it is open. I tried something like:

.offline, .online {
    background: rgba(51, 51, 51, 0.6);
}

or

.offline .open, .online .open {
    background: rgba(51, 51, 51, 0.6);
}

But neither is working. Any idea how to solve it?

Thank you so much!