gnome-control-center/panels/sound/output-test-wheel.css

wheel {
  background: alpha(currentColor, .08);
  border-radius: 100%;
  margin: 40px;
  min-width: 280px;
  min-height: 280px;
}

wheel > button {
  background-color: @window_bg_color;
  border: 1px solid @borders;
  padding: 3px;
}

wheel > button.playing {
  background-color: @accent_bg_color;
  color: @accent_fg_color;
}