Dark mode support for RCTPerfMonitor (#28130)

Summary:
Hi There 👋,

While I'm developing on iOS with dark appearance enabled, i notice that `Pref Monitor` view doesn't support dark mode yet, so here is the PR to fix it :)

## Changelog

[iOS] [Fixed] - Fix Pref Monitor in dark appearance
Pull Request resolved: #28130

Test Plan:
Run any React Native app on iOS > Turn on dark appearance
### Before
![Before](https://user-images.githubusercontent.com/4061838/74872974-8b75b600-535e-11ea-8550-763a598547ec.png)
### After
![After](https://user-images.githubusercontent.com/4061838/74872978-8ca6e300-535e-11ea-9862-4d3014e849f7.png)

Reviewed By: RSNara

Differential Revision: D20080019

Pulled By: PeteTheHeat

fbshipit-source-id: 9365daa3f7193a11760bc1372b8de2c3896def5c

Author: gorhom

React/CoreModules/RCTPerfMonitor.mm

@@ -197,12 +197,19 @@ - (UIView *)container
 {
   if (!_container) {
     _container = [[UIView alloc] initWithFrame:CGRectMake(10, 25, 180, RCTPerfMonitorBarHeight)];
-    _container.backgroundColor = UIColor.whiteColor;
     _container.layer.borderWidth = 2;
     _container.layer.borderColor = [UIColor lightGrayColor].CGColor;
     [_container addGestureRecognizer:self.gestureRecognizer];
     [_container addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self
                                                                              action:@selector(tap)]];
+    
+    _container.backgroundColor = [UIColor whiteColor];
+    #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && defined(__IPHONE_13_0) && \
+    __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_13_0
+    if (@available(iOS 13.0, *)) {
+      _container.backgroundColor = [UIColor systemBackgroundColor];
+    }
+    #endif
   }
 
   return _container;