fix: only set backgroundColor in default-app for default index.html (#28841)

Co-authored-by: Milan Burda <milan.burda@gmail.com>

default_app/default_app.ts

@@ -41,14 +41,14 @@ ipcMain.handle('bootstrap', (event) => {
   return isTrustedSender(event.sender) ? electronPath : null;
 });
 
-async function createWindow () {
+async function createWindow (backgroundColor?: string) {
   await app.whenReady();
 
   const options: Electron.BrowserWindowConstructorOptions = {
     width: 960,
     height: 620,
     autoHideMenuBar: true,
-    backgroundColor: '#2f3241',
+    backgroundColor,
     webPreferences: {
       preload: path.resolve(__dirname, 'preload.js'),
       contextIsolation: true,
@@ -96,7 +96,7 @@ export const loadURL = async (appUrl: string) => {
 };
 
 export const loadFile = async (appPath: string) => {
-  mainWindow = await createWindow();
+  mainWindow = await createWindow(appPath === 'index.html' ? '#2f3241' : undefined);
   mainWindow.loadFile(appPath);
   mainWindow.focus();
 };