<picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://assets.codepen.io/85188/tulip-dark.jpg">
  <img src="https://assets.codepen.io/85188/tulip-light.jpg" alt="">
</picture>

:root {
  --main-text: #333;
  --main-bg: #fff;
}

body {
  color: var(--main-text);
  background-color: var(--main-bg);
  padding: 2rem;
}
img {
  width: 100%;
}

@media (prefers-color-scheme: dark) {
  :root {
    --main-text: #ddd;
    --main-bg: #000;
  }
}