docs: add a mojave dark mode guide (#15267)

docs/tutorial/mojave-dark-mode-guide.md

@@ -0,0 +1,46 @@
+# Mojave Dark Mode
+
+In macOS 10.14 Mojave, Apple introduced a new [system-wide dark mode](https://developer.apple.com/design/human-interface-guidelines/macos/visual-design/dark-mode/)
+for all macOS computers.  By default Electron apps do not automatically adjust their UI and native interfaces
+to the dark mode setting when it's enabled. This is primarily due to Apple's own guidelines saying you **shouldn't**
+use the dark mode native interfaces if your app's own interfaces don't support dark mode themselves.
+
+If your app does have a dark mode, you can make your Electron app follow the system-wide dark mode setting.
+
+## Automatically updating the native interfaces
+
+"Native Interfaces" include the file picker, window border, dialogs, context menus and more; basically anything where
+the UI comes from macOS and not your app.  In order to make these interfaces update to dark mode automatically, you need
+to set the `NSRequiresAquaSystemAppearance` key in your app's `Info.plist` file to `false`.  E.g.
+
+```xml
+<plist>
+<dict>
+  ...
+  <key>NSRequiresAquaSystemAppearance</key>
+  <false />
+  ...
+</dict>
+</plist>
+```
+
+If you are using [`electron-packager` >= 12.2.0](https://github.com/electron-userland/electron-packager) or
+[`electron-forge` >= 6](https://github.com/electron-userland/electron-forge) you can set the
+[`darwinDarkModeSupport`](https://github.com/electron-userland/electron-packager/blob/master/docs/api.md#darwindarkmodesupport)
+option when packaging and this key will be set for you.
+
+## Automatically updating your own interfaces
+
+If your app has its own dark mode you should toggle it on and off in sync with the system's dark mode setting.  You can do
+this by listening for the theme changed event on Electron's `systemPreferences` module.  E.g.
+
+```js
+const { systemPreferences } = require('electron')
+
+systemPreferences.subscribeNotification(
+  'AppleInterfaceThemeChangedNotification',
+  function theThemeHasChanged () {
+    updateMyAppTheme(systemPreferences.isDarkMode())
+  }
+)
+```