<svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512">
  <style>
    @media (prefers-color-scheme: light) {
      .bg { fill: #0a0a0a; }
      .letter { fill: #fafafa; }
    }
    @media (prefers-color-scheme: dark) {
      .bg { fill: #fafafa; }
      .letter { fill: #0a0a0a; }
    }
  </style>
  <rect class="bg" width="512" height="512" rx="64" ry="64"/>
  <text class="letter" x="256" y="256" font-family="ui-serif, Georgia, 'Times New Roman', serif" font-size="360" font-weight="700" text-anchor="middle" dominant-baseline="central">O</text>
</svg>
