CODE EXAMPLE FOR HTML

dark mode favicon

/* HTML */
<link rel="icon" href="/favicon.svg" type="image/svg+xml">

/* SVG image */
<svg width="50" height="50" viewBox="0 0 50 50" xmlns="http://www.w3.org/2000/svg">
  <style>
    rect {
      fill: green;
    }
    @media (prefers-color-scheme: dark) {
      rect {
        fill: red;
      }
    }
  </style>
  <rect width="50" height="50" rx="5"/>
</svg>