Rounded corners on a gtk window

Question

I'm trying to get rounded corners on every corner of a window. How can I achieve that?

I tried to change my gtk.css in ~/.config/gtk-3.0/gtk.css and I'm getting rounded corners without transparency.

css I used:

.background.csd {
    border-radius: 0 0 8px 8px;
}
.background.maximized, .background.solid-csd {
    border-radius: 0;
}

Picture showing the corners I get