app/src/components/ThemeToggle.astro

---
import sunIconUrl from "../assets/images/sun.svg?url";
import moonIconUrl from "../assets/images/moon.svg?url";
---
<button id="theme-toggle" aria-label="Toggle color theme">
  <img class="icon light" src={sunIconUrl} alt="light" width="20" height="20" />
  <img class="icon dark" src={moonIconUrl} alt="dark" width="20" height="20" />
  <script>
    const btn = document.getElementById('theme-toggle');
    const media = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)');
    const prefersDark = media && media.matches;
    const saved = localStorage.getItem('theme'); // 'light' | 'dark' | null

    const apply = (mode) => {
      document.documentElement.dataset.theme = mode;
    };

    // Initial mode: localStorage overrides system preference
    apply(saved || (prefersDark ? 'dark' : 'light'));

    // If user hasn't chosen manually, follow system changes
    if (!saved && media) {
      const syncWithSystem = (e) => apply(e.matches ? 'dark' : 'light');
      if (media.addEventListener) media.addEventListener('change', syncWithSystem);
      else if (media.addListener) media.addListener(syncWithSystem);
    }

    btn.addEventListener('click', () => {
      const next = (document.documentElement.dataset.theme === 'dark') ? 'light' : 'dark';
      localStorage.setItem('theme', next);
      apply(next);
    });
  </script>
</button>