overleaf/services/web/frontend/stylesheets/bootstrap-5/pages/editor/review-panel.scss

:root {
  --review-icon: url('../../../../../public/img/ol-icons/review-icon-dark-theme.svg');
}

@include theme('light') {
  --review-icon: url('../../../../../public/img/ol-icons/review-icon-light-theme.svg');

  button {
    &.active,
    &:active {
      .review-icon {
        --review-icon: url('../../../../../public/img/ol-icons/review-icon-dark-theme.svg');
      }
    }
  }
}

.review-icon {
  display: inline-block;
  background-image: var(--review-icon);
  background-repeat: no-repeat;
  background-position: top;
  background-size: 30px;
  width: 30px;

  &::before {
    content: '\00a0'; // Non-breakable space. A non-breakable character here makes this icon work like font-awesome.
  }
}