Fix Appearance module to observe application appearance

[christophpurrer]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

This fixes the Appearance.getColorScheme() API to return the application-level appearance instead of the last appearance of any root view, which breaks down in multi-window applications.

In Messenger Desktop we always respect the OS appearance for main and setting windows, but force a darkTheme in the calling UI.
W/o this change returning from the calling UI would result in a wrong appearance setting on light theme OS setting.

Changelog

[macOS] [Fixed] - Fix Appearance module to observe application appearance

Test Plan

I have added this snippet to RNTester-macOS/ViewController.m

- (void)viewWillAppear {
    [super viewWillAppear];
    NSAppearance* appearance = [NSAppearance appearanceNamed:NSAppearanceNameVibrantLight];
    self.view.window.appearance = appearance;
}

The macOS appearance was set to darkTheme.
(The test plan is not ideal as I don't have a valid multi-window setup, but it should suffice for a basic test)

Before

b1.mov

After

a.mov

[Saadnajmi]

Thanks for the fix! Normally would ask to refactor the ifdef so that it's actually 2 methods calling a common block, but in the interest of time, I'll just merge this.