How can I change the "frame background" opacity?

[amadeogarcia]

Hello! I've managed to customize the sidebar a whole lot by using userChrome.css (mostly copying and tweaking tbh), to the point where, when extended (it retracts when I move my cursor away), it looks like this:

Now, I've achieved this by doing:

#sidebar-box[sidebarcommand="_3c078156-979c-498b-8990-85f7987dd929_-sidebar-action"] {
  opacity: 0.9;
  backdrop-filter: blur(5px);
}

What this means is that everything is made semi-transparent, even the tab preview, which is not what I want. I tried changing the "frame background" color in the Styles Editor with an rgba color, but it doesn't yield the desired effect.

P.S.: if anyone can explain to me what the sidebarcommand=.... is, I'll be grateful, as I copied that from the internet but have no idea what it does.

Thank you in advance!