blockscout/apps/block_scout_web/assets/css/components/_highlight.scss

//replace the default background color from highlightjs
.hljs {
  background: $gray-100;
}

.line-numbers {

  [data-line-number] {

    &:before {
      content: attr(data-line-number);
      display: inline-block;
      border-right: 1px solid $gray-400;
      padding: 0 .5em;
      margin-right: .5em;
      color: $gray-600
    }
  }
}