feat: add APIs to support mojave dark modes (#14755)

* feat: add APIs to support mojave dark mode

Closes #13387

* docs: fix system-prefs typo

lib/browser/api/system-preferences.js

@@ -1,5 +1,6 @@
 'use strict'
 
+const { app } = require('electron')
 const { EventEmitter } = require('events')
 const { systemPreferences, SystemPreferences } = process.atomBinding('system_preferences')
 
@@ -7,4 +8,40 @@ const { systemPreferences, SystemPreferences } = process.atomBinding('system_pre
 Object.setPrototypeOf(SystemPreferences.prototype, EventEmitter.prototype)
 EventEmitter.call(systemPreferences)
 
+if (process.platform === 'darwin') {
+  let appearanceTrackingSubscriptionID = null
+
+  systemPreferences.startAppLevelAppearanceTrackingOS = () => {
+    if (appearanceTrackingSubscriptionID !== null) return
+
+    const updateAppearanceBasedOnOS = () => {
+      const newAppearance = systemPreferences.isDarkMode()
+        ? 'dark'
+        : 'light'
+
+      if (systemPreferences.getAppLevelAppearance() !== newAppearance) {
+        systemPreferences.setAppLevelAppearance(newAppearance)
+        // TODO(MarshallOfSound): Once we remove this logic and build against 10.14
+        // SDK we should re-implement this event as a monitor of `effectiveAppearance`
+        systemPreferences.emit('appearance-changed', newAppearance)
+      }
+    }
+
+    appearanceTrackingSubscriptionID = systemPreferences.subscribeNotification(
+      'AppleInterfaceThemeChangedNotification',
+      updateAppearanceBasedOnOS
+    )
+
+    updateAppearanceBasedOnOS()
+  }
+
+  systemPreferences.stopAppLevelAppearanceTrackingOS = () => {
+    if (appearanceTrackingSubscriptionID === null) return
+
+    systemPreferences.unsubscribeNotification(appearanceTrackingSubscriptionID)
+  }
+
+  app.on('quit', systemPreferences.stopAppLevelAppearanceTrackingOS)
+}
+
 module.exports = systemPreferences