In that case I think that you must have initially made the individual panel list items have black background. But the panel list itself is grey, and the the panel behind it is now red. I would recommend you don’t set any background to the individual panel items (unless hovered, in case you want the to have custom hovered background), but rather only to the list or even make the list also have transparent so that the panel background is seen - that might go like this:

  panel-list[inxulpanel]{
    background: transparent !important;
    box-shadow: none !important;
  }

Put that also inside that about:preferences -moz-document block so it won’t accidentally affect any other pages than preferences.