discourse/app/assets/stylesheets/desktop/components/sidebar/sidebar-section.scss

.sidebar-section-wrapper {
  padding-block: 0.35rem;
  border-bottom: 1px solid var(--d-sidebar-border-color);
  &:first-child {
    padding-top: 0;
  }
  &[data-section-name="user-threads"] {
    padding-bottom: 0;
  }

  &:hover {
    .sidebar-section-header-wrapper {
      .btn.dropdown-select-box-header,
      .sidebar-section-header-button {
        opacity: 1;
      }
    }
  }

  .sidebar-section-header-wrapper {
    padding-right: calc(var(--d-sidebar-row-horizontal-padding) / 3);
  }
}