feat: add dark mode support

public/app.js

@@ -1,3 +1,36 @@
+function getPreferredTheme() {
+  const savedTheme = localStorage.getItem('theme');
+  if (savedTheme) {
+    return savedTheme;
+  }
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}
+
+function setTheme(theme) {
+  if (theme === 'dark') {
+    document.documentElement.setAttribute('data-theme', 'dark');
+    document.getElementById('theme-toggle').textContent = '☀️';
+  } else {
+    document.documentElement.removeAttribute('data-theme');
+    document.getElementById('theme-toggle').textContent = '🌙';
+  }
+  localStorage.setItem('theme', theme);
+}
+
+setTheme(getPreferredTheme());
+
+document.getElementById('theme-toggle').addEventListener('click', () => {
+  const currentTheme = document.documentElement.getAttribute('data-theme') === 'dark' ? 'dark' : 'light';
+  const newTheme = currentTheme === 'dark' ? 'light' : 'dark';
+  setTheme(newTheme);
+});
+
+window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', (e) => {
+  if (!localStorage.getItem('theme')) {
+    setTheme(e.matches ? 'dark' : 'light');
+  }
+});
+
 // Navigation
 const navLinks = document.querySelectorAll('.nav-link');
 const pages = document.querySelectorAll('.page');