Implement theme toggle functionality and refactor styles for light/dark mode support

src/App.tsx

@@ -1,5 +1,6 @@
 import { useTranslation } from 'react-i18next'
 import LanguageSelector from './components/LanguageSelector'
+import ThemeToggle from './components/ThemeToggle'
 import './App.css'
 
 function App() {
@@ -34,6 +35,7 @@ function App() {
             <a href="#blog">{t('nav.blog')}</a>
             <a href="#git">{t('nav.git')}</a>
             <a href="#forum">{t('nav.forum')}</a>
+            <ThemeToggle />
             <LanguageSelector />
             <button className="join-btn">{t('nav.joinNow')}</button>
           </div>