feat(platform-utils): Get and react to changes to the system theme (#161)

These changes will allow the WebExtension (and later the desktop application) to respect the system theme.

I've added the Electron implementation until I realized that the required API [has been implemented but not released yet](https://www.electronjs.org/docs/api/native-theme/history). Let me know if you I should remove the code.

Part of https://github.com/bitwarden/browser/issues/1256.

https://www.electronjs.org/docs/api/native-theme

Co-authored-by: Chad Scharf <3904944+cscharf@users.noreply.github.com>

src/electron/services/electronPlatformUtils.service.ts

@@ -3,6 +3,7 @@ import {
     ipcRenderer,
     remote,
     shell,
+    // nativeTheme,
 } from 'electron';
 import * as fs from 'fs';
 
@@ -220,6 +221,17 @@ export class ElectronPlatformUtilsService implements PlatformUtilsService {
         });
     }
 
+    getDefaultSystemTheme() {
+        return 'light' as 'light' | 'dark';
+        // return nativeTheme.shouldUseDarkColors ? 'dark' : 'light';
+    }
+
+    onDefaultSystemThemeChange(callback: ((theme: 'light' | 'dark') => unknown)) {
+        // nativeTheme.on('updated', () => {
+        //     callback(this.getDefaultSystemTheme());
+        // });
+    }
+
     supportsSecureStorage(): boolean {
         return true;
     }