**Is your feature request related to a problem? Please describe.**
Electron has an API which works on macOS for detecting when the system is in Dark Mode.
[Windows 10 also has a dark mode setting](https://www.digitaltrends.com/computing/how-to-enable-dark-mode-in-windows-10/). It would be useful if the same Electron API also worked on Windows.

**Describe the solution you'd like**
1. The `systemPreferences.isDarkMode()` method should work on Windows.
2. Ideally a way of subscribing to change events, equivalent to `systemPreferences.subscribeNotification('AppleInterfaceThemeChangedNotification', callback)`. Or perhaps the `color-changed` event (or another event) would be fired by `systemPreferences`.

**Additional context**
On Windows the state of the dark mode setting [is stored in the](https://stackoverflow.com/a/51336913/365069) `HKCU(HKLM)\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize\AppsUseLightTheme` registry key.

shell/browser/api/atom_api_system_preferences.cc
#if defined(OS_WIN)
  Browser::Get()->RemoveObserver(this);
#endif
}
#if !defined(OS_MACOSX)
bool SystemPreferences::IsDarkMode() {
  return false;
}
#endif
bool SystemPreferences::IsInvertedColorScheme() {
  return color_utils::IsInvertedColorScheme();
}
#if !defined(OS_WIN)
bool SystemPreferences::IsHighContrastColorScheme() {
  return false;
}
#endif  
v8::Local<v8::Value> SystemPreferences::GetAnimationSettings(
    v8::Isolate* isolate) {
  mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
  dict.SetHidden("simple", true);
  dict.Set("shouldRenderRichAnimation",
           gfx::Animation::ShouldRenderRichAnimation());
  dict.Set("scrollAnimationsEnabledBySystem",
           gfx::Animation::ScrollAnimationsEnabledBySystem());
  dict.Set("prefersReducedMotion", gfx::Animation::PrefersReducedMotion());
  return dict.GetHandle();
}
mate::Handle<SystemPreferences> SystemPreferences::Create(