<div class="module">
  Lorem ipsum dolor sit, amet consectetur adipisicing elit. Sequi magnam quos, reprehenderit voluptas minima animi aliquid modi aut accusantium inventore tempora iusto similique unde! Necessitatibus quidem recusandae perspiciatis nostrum quam.
</div>

<div class="module dark">
  Lorem ipsum dolor sit, amet consectetur adipisicing elit. Sequi magnam quos, reprehenderit voluptas minima animi aliquid modi aut accusantium inventore tempora iusto similique unde! Necessitatibus quidem recusandae perspiciatis nostrum quam.
</div>
:root {
  --ON: initial;
  --OFF: ;
}

.module {
  --dark: var(--OFF);
  padding: 1rem;

  background: var(--dark, black);
  color: var(--dark, white);
  letter-spacing: var(--dark, 0.2px);
  border: var(--dark, 3px solid #888);
}

.dark {
  --dark: var(--ON);
}
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