Ion-toolbar color not changeable in Internet Explorer

I changed my toolbar background (ionic 4) to transparent using:

ion-toolbar {
     --background: transparent !important;
     --ion-color-base: transparent !important;

This works on all browsers except IE (Im tested on IE11).

I also tried setting the background with a selector that works when I edit it directly in IE, but it just gets ignored when putting it in my CSS:

ion-toolbar .toolbar-background {
      background: rgba(0,0,0,0) !important;

Is there any way to apply a workaround?