Free dark mode toggle for any static site
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1. Add dark-mode.js to your site and include it with <script src="dark-mode.js"></script>.
2. Add a button with id="toggle" anywhere on the page.
3. Define your dark colors under a [data-theme="dark"] CSS selector (see demo.html for the pattern).

Respects a saved choice across visits (localStorage), and falls back to the visitor's OS/browser
preference when they haven't chosen yet.
