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

.darkmode {
  position: relative;
  width: 20px;
  height: 20px;
  margin: 1rem;

  & > .toggle {
    display: none;
    box-sizing: border-box;
  }

  & svg {
    cursor: pointer;
    opacity: 0;
    position: absolute;
    width: 20px;
    height: 20px;
    top: calc(50% - 10px);
    fill: var(--darkgray);
    transition: opacity 0.1s ease;
  }
}

:root[saved-theme="dark"] .toggle ~ label {
  & > #dayIcon {
    opacity: 0;
  }
  & > #nightIcon {
    opacity: 1;
  }
}

:root .toggle ~ label {
  & > #dayIcon {
    opacity: 1;
  }
  & > #nightIcon {
    opacity: 0;
  }
}