I have a very dark panel in XFCE and xfce4-pulseaudio-plugin 0.2.4 (Debian Stretch) looks really ugly with the almost white background sqare. I also tried to edit the svg-icons supplied with he package in /usr/share/icons/hicolor/scalable/status/audio-volume-??-symbolic.svg without any success. My whisch: background transparent and color of volume indicator selectable/reversible. Seems to be hard coded :-(