docs/_static/custom.css

/* Use the full viewport on ordinary desktop displays, where Furo's fixed
   content width leaves little room after both sidebars. Once the content area
   reaches 90em (84em plus padding), grow symmetric outer margins instead. */
@media (min-width: 67em) {
  .page .sidebar-drawer {
    width: max(15em, calc(50% - 45em));
  }

  .main > .content {
    width: min(84em, calc(100vw - 36em));
  }
}

/* Furo moves the page-local TOC into a drawer below 82em, freeing its 15em
   column for the main content. */
@media (min-width: 67em) and (max-width: 82em) {
  .main > .content {
    width: calc(100vw - 21em);
  }
}