~/.config/gtk-3.0/gtk.css (Xfce Panel Styling)

gtk.css

/* ~/.config/gtk-3.0/gtk.css */
@import 'colors.css';

.xfce4-panel {
    background: rgba(255, 255, 255, 0.1);
    border: none;
    border-radius: 4px;
}

.tasklist button {
    background: transparent;
    border: none;
}

#pulseaudio-button > image {
    -gtk-icon-transform: scale(.7);
}

#xfce4-clipman-plugin > image {
    -gtk-icon-transform: scale(.7);
}

#xfce4-notification-plugin > image {
   -gtk-icon-transform: scale(.7);
}