quartz-research-note/quartz/components/styles/toc.scss

button#toc {
  background-color: transparent;
  border: none;
  text-align: left;
  cursor: pointer;
  padding: 0;
  color: var(--dark);
  display: flex;
  align-items: center;

  & h3 {
    font-size: 1rem;
    display: inline-block;
    margin: 0;
  }

  & .fold {
    margin-left: 0.5rem; 
    transition: transform 0.3s ease;
    opacity: 0.8;
  }

  &.collapsed .fold {
    transform: rotateZ(-90deg)
  }
}
  
#toc-content {
  list-style: none;
  overflow: hidden;
  max-height: none;
  transition: max-height 0.5s ease;

  &.collapsed > .overflow::after {
    opacity: 0;
  }

  & ul {
    list-style: none;
    margin: 0.5rem 0;
    padding: 0;
    & > li > a {
      color: var(--dark);
      opacity: 0.35;
      transition: 0.5s ease opacity, 0.3s ease color;
      &.in-view {
        opacity: 0.75;
      }
    }
  }

  @for $i from 0 through 6 {
    & .depth-#{$i} {
      padding-left: calc(1rem * #{$i});
    }
  }
}