docs: improve dark mode example fiddle (#36596)

docs/fiddles/features/dark-mode/main.js

@@ -0,0 +1,43 @@
+const { app, BrowserWindow, ipcMain, nativeTheme } = require('electron')
+const path = require('path')
+
+function createWindow () {
+  const win = new BrowserWindow({
+    width: 800,
+    height: 600,
+    webPreferences: {
+      preload: path.join(__dirname, 'preload.js')
+    }
+  })
+
+  win.loadFile('index.html')
+}
+
+ipcMain.handle('dark-mode:toggle', () => {
+  if (nativeTheme.shouldUseDarkColors) {
+    nativeTheme.themeSource = 'light'
+  } else {
+    nativeTheme.themeSource = 'dark'
+  }
+  return nativeTheme.shouldUseDarkColors
+})
+
+ipcMain.handle('dark-mode:system', () => {
+  nativeTheme.themeSource = 'system'
+})
+
+app.whenReady().then(() => {
+  createWindow()
+
+  app.on('activate', () => {
+    if (BrowserWindow.getAllWindows().length === 0) {
+      createWindow()
+    }
+  })
+})
+
+app.on('window-all-closed', () => {
+  if (process.platform !== 'darwin') {
+    app.quit()
+  }
+})