Make sure not to override user background color (#36215)

Summary:
Pull Request resolved: #36215

When we introduced the RCTAppDelegate library, we prepared some template methods for the user to customise their views.

However, after they customized their view, we were chaing the background color to match the system background. This would actually override the background color they set in their own customisation step.

This change make sure that we set the background color before they apply their customisations. In this way, we set the background color and, if they want, they can change it and that changw would be honoured.

This change also fixes [this issue](#35937)

## Changelog
[iOS][Fixed] - Honour background color customisation in RCTAppDelegate

Reviewed By: cortinico

Differential Revision: D43435946

fbshipit-source-id: cdbdbd5b07082ae7843a4dab352dd1195c69e036

Libraries/AppDelegate/RCTAppDelegate.mm

@@ -54,12 +54,6 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   NSDictionary *initProps = [self prepareInitialProps];
   UIView *rootView = [self createRootViewWithBridge:self.bridge moduleName:self.moduleName initProps:initProps];
 
-  if (@available(iOS 13.0, *)) {
-    rootView.backgroundColor = [UIColor systemBackgroundColor];
-  } else {
-    rootView.backgroundColor = [UIColor whiteColor];
-  }
-
   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
   UIViewController *rootViewController = [self createRootViewController];
   rootViewController.view = rootView;
@@ -101,7 +95,14 @@ - (UIView *)createRootViewWithBridge:(RCTBridge *)bridge
 #if RCT_NEW_ARCH_ENABLED
   enableFabric = self.fabricEnabled;
 #endif
-  return RCTAppSetupDefaultRootView(bridge, moduleName, initProps, enableFabric);
+  UIView *rootView = RCTAppSetupDefaultRootView(bridge, moduleName, initProps, enableFabric);
+  if (@available(iOS 13.0, *)) {
+    rootView.backgroundColor = [UIColor systemBackgroundColor];
+  } else {
+    rootView.backgroundColor = [UIColor whiteColor];
+  }
+
+  return rootView;
 }
 
 - (UIViewController *)createRootViewController