converse.js/sass/inverse/_headline.scss

#conversejs.fullscreen {
    .chatbox.headlines {
        .box-flyout {
            background-color: $headline-head-color;
        }
        .chat-head {
            &.chat-head-chatbox {
                background-color: $headline-head-color;
            }
        }
        .flyout {
            border: $flyout-padding solid $headline-head-color;
            border-top: 0.8em solid $headline-head-color;
        }
    }
}