feat: improve theme system with cookie and prefers-color-scheme

static/theme.ts

@@ -1,18 +1,65 @@
 type Theme = 'light' | 'dark';
 
 const STORAGE_KEY = 'theme';
+const COOKIE_KEY = 'theme';
+
+const getLocalStorage = (): Storage | null => {
+  try {
+    return window.localStorage;
+  } catch {
+    return null;
+  }
+};
+
+const readCookie = (key: string): string | null => {
+  const entries = document.cookie.split(';').map(part => part.trim());
+  for (const entry of entries) {
+    if (!entry) continue;
+    const eqIndex = entry.indexOf('=');
+    if (eqIndex === -1) continue;
+    const k = entry.slice(0, eqIndex).trim();
+    if (k !== key) continue;
+    return decodeURIComponent(entry.slice(eqIndex + 1));
+  }
+  return null;
+};
+
+const writeCookie = (key: string, value: string): void => {
+  const maxAgeSeconds = 60 * 60 * 24 * 365;
+  document.cookie = `${encodeURIComponent(key)}=${encodeURIComponent(value)}; Max-Age=${maxAgeSeconds}; Path=/; SameSite=Lax`;
+};
+
+const getPreferredTheme = (): Theme => {
+  const prefersDark = window.matchMedia?.('(prefers-color-scheme: dark)')?.matches ?? false;
+  return prefersDark ? 'dark' : 'light';
+};
+
+const normalizeTheme = (raw: string | null): Theme | null => {
+  if (raw === 'light' || raw === 'dark') return raw;
+  return null;
+};
 
 const getSavedTheme = (): Theme => {
-  const raw = localStorage.getItem(STORAGE_KEY);
-  if (raw === 'light' || raw === 'dark') {
-    return raw;
-  }
-  return 'dark'; // default to dark
+  const storage = getLocalStorage();
+  const fromStorage = normalizeTheme(storage?.getItem(STORAGE_KEY) ?? null);
+  if (fromStorage) return fromStorage;
+
+  const fromCookie = normalizeTheme(readCookie(COOKIE_KEY));
+  if (fromCookie) return fromCookie;
+
+  return getPreferredTheme();
 };
 
 const applyTheme = (theme: Theme): void => {
   document.documentElement.setAttribute('data-theme', theme);
-  localStorage.setItem(STORAGE_KEY, theme);
+  document.documentElement.style.colorScheme = theme;
+  const storage = getLocalStorage();
+  try {
+    storage?.setItem(STORAGE_KEY, theme);
+  } catch {
+    // ignore
+  }
+  writeCookie(COOKIE_KEY, theme);
 };
 
 const cycleTheme = (): void => {
@@ -27,11 +74,11 @@ const initTheme = (): void => {
 
   // delegated click handler on theme buttons
   document.addEventListener('click', e => {
-    const target = e.target as HTMLElement;
-    const btn = target.closest('#theme-toggle, #footer-theme-toggle') as HTMLButtonElement | null;
-    if (btn) {
-      cycleTheme();
-    }
+    const target = e.target;
+    if (!(target instanceof Element)) return;
+    const btn = target.closest('#theme-toggle, #footer-theme-toggle');
+    if (!(btn instanceof HTMLButtonElement)) return;
+    cycleTheme();
   });
 };