::-webkit-scrollbar CSS pseudo-element

.visible-scrollbar,
.invisible-scrollbar,
.mostly-customized-scrollbar {
  display: block;
  width: 10em;
  overflow: auto;
  height: 2em;
  padding: 1em;
  margin: 1em auto;
  outline: 2px dashed cornflowerblue;
}

.invisible-scrollbar::-webkit-scrollbar {
  display: none;
}

/* Demonstrate a "mostly customized" scrollbar
 * (won't be visible otherwise if width/height is specified) */
.mostly-customized-scrollbar::-webkit-scrollbar {
  width: 5px;
  height: 8px;
  background-color: #aaaaaa; /* or add it to the track */
}

/* Add a thumb */
.mostly-customized-scrollbar::-webkit-scrollbar-thumb {
  background: black;
}

<div class="visible-scrollbar">
  <h3>Visible scrollbar</h3>
  <p>
    Etiam sagittis sem sed lacus laoreet, eu fermentum eros auctor. Proin at
    nulla elementum, consectetur ex eget, commodo ante. Sed eros mi, bibendum ut
    dignissim et, maximus eget nibh. Phasellus blandit quam turpis, at mollis
    velit pretium ut. Nunc consequat efficitur ultrices. Nullam hendrerit
    posuere est. Nulla libero sapien, egestas ac felis porta, cursus ultricies
    quam. Vestibulum tincidunt accumsan sapien, a fringilla dui semper in.
    Vivamus consectetur ipsum a ornare blandit. Aenean tempus at lorem sit amet
    faucibus. Curabitur nibh justo, faucibus sed velit cursus, mattis cursus
    dolor. Pellentesque id pretium est. Quisque convallis nisi a diam malesuada
    mollis. Aliquam at enim ligula.
  </p>
</div>

<div class="invisible-scrollbar">
  <h3>Invisible scrollbar</h3>
  <p>
    Thisisaveeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeerylongword
  </p>
</div>

<div class="mostly-customized-scrollbar">
  <h3>Custom scrollbar</h3>
  <p>
    Thisisaveeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeerylongword<br />
    And pretty tall<br />
    thing with weird scrollbars.<br />
    Who thought scrollbars could be made weird?
  </p>
</div>

<div class="scroll-box">
  <h1>Yoshi</h1>
  <p>
    Yoshi is a fictional dinosaur who appears in video games published by
    Nintendo. Yoshi debuted in Super Mario World (1990) on the SNES as Mario and
    Luigi's sidekick.
  </p>
  <p>
    Throughout the mainline Super Mario series, Yoshi typically serves as
    Mario's trusted steed.
  </p>
  <p>
    With a gluttonous appetite, Yoshi can gobble enemies with his long tongue,
    and lay eggs that doubly function as projectiles.
  </p>
</div>

.scroll-box {
  overflow: auto;
  width: 20rem;
  height: 5rem;
  border: 2px solid cornflowerblue;
  margin: 2rem auto;
  font-family: monospace;
}

/* For browsers that support `scrollbar-*` properties */
@supports (scrollbar-color: auto) {
  .scroll-box {
    scrollbar-color: aquamarine cornflowerblue;
  }
}

/* Otherwise, use `::-webkit-scrollbar-*` pseudo-elements */
@supports selector(::-webkit-scrollbar) {
  .scroll-box::-webkit-scrollbar {
    background: aquamarine;
  }
  .scroll-box::-webkit-scrollbar-thumb {
    background: cornflowerblue;
  }
}