Styling the ::before element on a mat-expansion-panel-header?

In this demo I’m trying to style the mat-expansion-panel-header‘s ::before pseudo element like this (Style rules added to styles.scss):

mat-expansion-panel-header {
  position: relative;
}

mat-expansion-panel-header::before {
  content: '';
  position: absolute;
  top: 0px;
  left: 0px;
  bottom: 0px;
  right: 0px;
  z-index: 1000;
  background-color: black;
}

So if the above rule would take effect the mat-expansion-panel-header items in the demo would just be black.

However the rule does not take effect and when I look in the developer tools I don’t see the ::before element.

Any ideas on how to fix this?