wagtail/client/scss/components/_progressbar.scss

.progress {
  border-radius: 1.2em;
  background-color: theme('colors.surface-button-hover');
  border: 1px solid theme('colors.surface-button-default');
  opacity: 0;

  &.active {
    @include transition(opacity 0.3s ease);
    opacity: 1;
  }

  .bar {
    @include transition(width 0.3s ease);
    border-radius: 1.5em;
    overflow: hidden;
    text-align: end;
    line-height: 1.2em;
    color: theme('colors.text-button');
    font-size: 0.85em;
    background-color: theme('colors.surface-button-default');
    height: 1.2em;
    padding-inline-end: 1em;

    @media (forced-colors: active) {
      background-color: ButtonText;
    }
  }
}