<img src="https://assets.codepen.io/85188/tulip.jpg" alt="" />

:root {
  --main-bg: #fff;
}
body {
  background-color: var(--main-bg);
  padding: 2rem;
}
img {
  width: 100%;
}
@media (prefers-color-scheme: dark) {
  :root {
    --main-bg: #000;
  }
  img {
    filter: saturate(70%) brightness(50%);
  }
}