storybook/lib/ui/src/components/sidebar/SidebarSimpleBarStyles.js

// manually import CSS instead of using "import 'simplebar/dist/simplebar.css';"
// customize scrollbar here
import { css } from '@storybook/theming';

// export const simplebarStyles = css`
export const getSimpleBarStyles = theme => css`
  [data-simplebar] {
    position: relative;
    flex-direction: column;
    flex-wrap: wrap;
    justify-content: flex-start;
    align-content: flex-start;
    align-items: flex-start;
    width: inherit;
    height: inherit;
    max-width: inherit;
    max-height: inherit;
  }

  .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 {
    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;
    visibility: visible;
    overflow: scroll; /* Scroll on this element otherwise element can't have a padding applied properly */
    max-width: 100%; /* Not required for horizontal scroll to trigger */
    max-height: 100%; /* Needed for vertical scroll to trigger */
  }

  .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: inherit;
    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;
  }

  .simplebar-scrollbar {
    position: absolute;
    right: 2px;
    width: 6px;
    min-height: 10px;
  }

  .simplebar-scrollbar:before {
    position: absolute;
    content: '';
    background: ${theme.base === 'light' ? theme.color.darkest : theme.color.lightest};
    border-radius: 6px;
    left: 0;
    right: 0;
    opacity: 0;
    transition: opacity 0.2s linear;
  }

  .simplebar-track .simplebar-scrollbar.simplebar-visible:before {
    /* When hovered, remove all transitions from drag handle */
    opacity: 0.2;
    transition: opacity 0s linear;
  }

  .simplebar-track.simplebar-vertical {
    top: 0;
    width: 10px;
    right: 4px;
  }

  .simplebar-track.simplebar-vertical .simplebar-scrollbar:before {
    top: 2px;
    bottom: 2px;
  }

  .simplebar-track.simplebar-horizontal {
    left: 0;
    height: 10px;
    bottom: 0;
  }

  .simplebar-track.simplebar-horizontal .simplebar-scrollbar:before {
    height: 100%;
    left: 2px;
    right: 2px;
  }

  .simplebar-track.simplebar-horizontal .simplebar-scrollbar {
    right: auto;
    left: 0;
    top: 2px;
    height: 6px;
    min-height: 0;
    min-width: 10px;
    width: auto;
  }

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

  .hs-dummy-scrollbar-size {
    direction: rtl;
    position: fixed;
    opacity: 0;
    visibility: hidden;
    height: 500px;
    width: 500px;
    overflow-y: hidden;
    overflow-x: scroll;
  }
`;