discourse/app/assets/stylesheets/common/components/composer-toggle-switch.scss

.composer-toggle-switch {
  --toggle-switch-width: 40px;
  --toggle-switch-height: 24px;
  height: 100%;
  grid-column: span 2;
  justify-content: center;
  display: flex;
  align-items: center;
  border: 0;
  padding: 0;
  background: transparent;

  &:focus-visible {
    outline: none;
  }

  &[disabled] {
    opacity: 0.5;
    cursor: not-allowed;
  }

  &__slider {
    display: inline-block;
    background: var(--primary-low);
    width: var(--toggle-switch-width);
    height: var(--toggle-switch-height);
    position: relative;
    vertical-align: middle;
    border-radius: 0.25em;

    :focus-visible & {
      outline: 2px solid var(--tertiary);
      outline-offset: 2px;
    }

    &::before {
      content: "";
      display: block;
      position: absolute;
      background-color: var(--tertiary-low);
      width: calc(var(--toggle-switch-height) - 2px);
      height: calc(var(--toggle-switch-height) - 4px);
      top: 2px;
      transition:
        left 0.25s,
        right 0.25s;
      border-radius: 0.25em;
      box-shadow: 0 1px 3px 1px rgba(0, 0, 0, 0.1);

      .--markdown & {
        left: 2px;
      }

      .--rte & {
        right: 2px;
      }

      @media (prefers-reduced-motion: reduce) {
        transition-duration: 0ms;
      }
    }
  }

  &__left-icon,
  &__right-icon {
    display: inline-block;
    position: absolute;
    opacity: 0;
    transition:
      opacity 0.25s left 0.25s,
      right 0.25s;
    height: 100%;
    width: calc(var(--toggle-switch-height) - 2px);

    @media (prefers-reduced-motion: reduce) {
      transition-duration: 0ms;
    }

    .--markdown & {
      left: 2px;
    }

    .--rte & {
      right: 2px;
    }

    &.--active {
      opacity: 1;
    }

    .d-icon {
      font-size: var(--font-down-1);
      color: var(--primary);
      vertical-align: text-bottom;
    }
  }
}