Create index.js

js/src/forum/index.js

@@ -0,0 +1,13 @@
+import { extend } from 'flarum/common/extend';
+import SettingsPage from 'flarum/forum/components/SettingsPage';
+import ThemeSwitcher from './components/ThemeSwitcher';
+
+app.initializers.add('demo-theme-switcher', () => {
+  extend(SettingsPage.prototype, 'settingsItems', function (items) {
+    items.add(
+      'theme-switcher',
+      <ThemeSwitcher />,
+      10 // 排序，越小越靠上
+    );
+  });
+});