converse.js/sass/_bookmarks.scss

#conversejs.fullscreen {
    #controlbox {
        #chatrooms {
            .bookmarks-list {
                dl.rooms-list.bookmarks {
                    dd.available-chatroom {
                        a {
                            &.open-room {
                                width: 80%;
                            }
                        }
                    }
                }
            }
        }
    }
}