use os detection for initial dark mode setting

src/settings/index.ts

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2021 The Pybricks Authors
 
+import { prefersDarkMode } from '../utils/os';
+
 // Definitions for user settings.
 
 export enum SettingId {
@@ -14,6 +16,7 @@ export function getDefaultBooleanValue(id: SettingId): boolean {
         case SettingId.ShowDocs:
             return window.innerWidth >= 1024;
         case SettingId.DarkMode:
+            return prefersDarkMode();
         case SettingId.FlashCurrentProgram:
             return false;
         // istanbul ignore next: it is a programmer error if we hit this