スクロールバーをSimpleBarを使用して実装する

// 最新版
npm i simplebar

// バージョン指定
npm i simplebar@5.3.6

// --------------------------

// lib | simplebar.css

// --------------------------
[data-simplebar] {
  position: relative;
  flex-direction: column;
  flex-wrap: wrap;
  justify-content: flex-start;
  align-content: flex-start;
  align-items: flex-start;
}

.simplebar-wrapper {
  overflow: hidden;
  width: inherit;
  height: inherit;
  max-width: inherit;
  max-height: inherit;
}

.simplebar-mask {
  direction: inherit;
  position: absolute;
  overflow: hidden;
  padding: 0;
  margin: 0;
  left: 0;
  top: 0;
  bottom: 0;
  right: 0;
  width: auto !important;
  height: auto !important;
  z-index: 0;
}

.simplebar-offset {
  direction: inherit !important;
  box-sizing: inherit !important;
  resize: none !important;
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  padding: 0;
  margin: 0;
  -webkit-overflow-scrolling: touch;
}

.simplebar-content-wrapper {
  direction: inherit;
  box-sizing: border-box !important;
  position: relative;
  display: block;
  height: 100%; /* Required for horizontal native scrollbar to not appear if parent is taller than natural height */
  width: auto;
  max-width: 100%; /* Not required for horizontal scroll to trigger */
  max-height: 100%; /* Needed for vertical scroll to trigger */
  overflow: auto;
  scrollbar-width: none;
  -ms-overflow-style: none;
}

.simplebar-content-wrapper::-webkit-scrollbar,
.simplebar-hide-scrollbar::-webkit-scrollbar {
  display: none;
  width: 0;
  height: 0;
}

.simplebar-content:before,
.simplebar-content:after {
  content: " ";
  display: table;
}

.simplebar-placeholder {
  max-height: 100%;
  max-width: 100%;
  width: 100%;
  pointer-events: none;
}

.simplebar-height-auto-observer-wrapper {
  box-sizing: inherit !important;
  height: 100%;
  width: 100%;
  max-width: 1px;
  position: relative;
  float: left;
  max-height: 1px;
  overflow: hidden;
  z-index: -1;
  padding: 0;
  margin: 0;
  pointer-events: none;
  flex-grow: inherit;
  flex-shrink: 0;
  flex-basis: 0;
}

.simplebar-height-auto-observer {
  box-sizing: inherit;
  display: block;
  opacity: 0;
  position: absolute;
  top: 0;
  left: 0;
  height: 1000%;
  width: 1000%;
  min-height: 1px;
  min-width: 1px;
  overflow: hidden;
  pointer-events: none;
  z-index: -1;
}

.simplebar-track {
  z-index: 1;
  position: absolute;
  right: 0;
  bottom: 0;
  pointer-events: none;
  overflow: hidden;
}

[data-simplebar].simplebar-dragging {
  pointer-events: none;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

[data-simplebar].simplebar-dragging .simplebar-content {
  pointer-events: none;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

[data-simplebar].simplebar-dragging .simplebar-track {
  pointer-events: all;
}

.simplebar-scrollbar {
  position: absolute;
  left: 0;
  right: 0;
  min-height: 10px;
}

.simplebar-scrollbar:before {
  position: absolute;
  content: "";
  background: black;
  border-radius: 7px;
  left: 2px;
  right: 2px;
  opacity: 0;
  transition: opacity 0.2s 0.5s linear;
}

.simplebar-scrollbar.simplebar-visible:before {
  opacity: 0.5;
  transition-delay: 0s;
  transition-duration: 0s;
}

.simplebar-track.simplebar-vertical {
  top: 0;
  width: 11px;
}

.simplebar-scrollbar:before {
  top: 2px;
  bottom: 2px;
  left: 2px;
  right: 2px;
}

.simplebar-track.simplebar-horizontal {
  left: 0;
  height: 11px;
}

.simplebar-track.simplebar-horizontal .simplebar-scrollbar {
  right: auto;
  left: 0;
  top: 0;
  bottom: 0;
  min-height: 0;
  min-width: 10px;
  width: auto;
}

/* Rtl support */
[data-simplebar-direction="rtl"] .simplebar-track.simplebar-vertical {
  right: auto;
  left: 0;
}

.simplebar-dummy-scrollbar-size {
  direction: rtl;
  position: fixed;
  opacity: 0;
  visibility: hidden;
  height: 500px;
  width: 500px;
  overflow-y: hidden;
  overflow-x: scroll;
  -ms-overflow-style: scrollbar !important;
}

.simplebar-dummy-scrollbar-size > div {
  width: 200%;
  height: 200%;
  margin: 10px 0;
}

.simplebar-hide-scrollbar {
  position: fixed;
  left: 0;
  visibility: hidden;
  overflow-y: scroll;
  scrollbar-width: none;
  -ms-overflow-style: none;
}

import SimpleBar from "simplebar";

export function SetSimplebar() {
  const simplebarList = [];
  const simplebarElementList = [...document.querySelectorAll(".js-simplebar")];
  if (simplebarElementList && simplebarElementList.length > 0) {
    simplebarElementList.forEach((ele) => {
      const obj = new SimpleBar(ele);
      simplebarList.push(obj);
    });
  }
}

 <div class="l-container">
  <div class="c-privacy js-simplebar" data-simplebar data-simplebar-auto-hide="false">
    <div class="c-privacy__inner">
      コンテンツ
    </div>
  </div>
</div>

// --------------------------

// lib simplebar 上書き

// --------------------------
// スクロールバーのボール
.simplebar-scrollbar {
  top: 0px;
  background: #707070;
  border-radius: 10px;
  padding: 1px 0;
  overflow: hidden;
}
.simplebar-scrollbar::before {
  background: #707070;
  width: 100%;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

// /スクロールバーの背景
.simplebar-track {
  border-radius: 10px;
  background: #e9e9e9;
}

// スクロールバーを常に表示する
.simplebar-scrollbar.simplebar-visible::before {
  opacity: 1;
}

// 縦スクロールバーの位置
.simplebar-track.simplebar-vertical {
  top: 5%;
  right: 16px;
  height: 90%;
  width: 4px;
}

<div class="l-container">
  <div class="c-horizontal js-simplebar" data-simplebar data-simplebar-auto-hide="false">
    <div class="c-horizontal__inner">
      コンテンツ
    </div>
  </div>
</div>

// コンテンツを横並びにする
.c-horizontal__inner {
  display: flex;
}

// --------------------------

// lib simplebar 上書き

// --------------------------
// スクロールバーのボールの調整
.simplebar-scrollbar {
  top: 0px;
  background: #707070;
  border-radius: 10px;
  padding: 1px 0;
  overflow: hidden;
}
.simplebar-scrollbar::before {
  background: #707070;
  width: 100%;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

// スクロールバーの背景
.simplebar-track {
  border-radius: 10px;
  background: #e9e9e9;
}

// スクロールバーを常に表示
.simplebar-scrollbar.simplebar-visible::before {
  opacity: 1;
}

// 横スクロールバーの調整
.simplebar-track.simplebar-horizontal {
  height: 6px;
  left: 3%;
  width: 94%;
}