style: improve docs branding and header

src/components/SystemTheme.astro

@@ -0,0 +1,20 @@
+<script is:inline>
+  (() => {
+    const preference = window.matchMedia('(prefers-color-scheme: dark)');
+
+    const applySystemTheme = () => {
+      const theme = preference.matches ? 'dark' : 'light';
+      document.documentElement.dataset.theme = theme;
+      document.documentElement.style.colorScheme = theme;
+    };
+
+    try {
+      localStorage.removeItem('starlight-theme');
+    } catch {
+      // The system preference still works when storage is unavailable.
+    }
+
+    applySystemTheme();
+    preference.addEventListener?.('change', applySystemTheme);
+  })();
+</script>