src/theme/DocRoot/Layout/Sidebar/ExpandButton/styles.module.css

@media (min-width: 997px) {
  .expandButton {
    position: absolute;
    top: 0;
    right: 0;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background-color var(--ifm-transition-fast) ease;
    background-color: var(--docusaurus-collapse-button-bg);
  }

  .expandButton:hover,
  .expandButton:focus {
    background-color: var(--docusaurus-collapse-button-bg-hover);
  }

  .expandButtonIcon {
    transform: rotate(0);
  }

  [dir='rtl'] .expandButtonIcon {
    transform: rotate(180deg);
  }
}