use os detection for initial dark mode setting

src/utils/os.test.ts

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2021 The Pybricks Authors
 
-import { isMacOS } from './os';
+import { isMacOS, prefersDarkMode } from './os';
 
 describe('isMacOS', () => {
     test('is true', () => {
@@ -13,3 +13,22 @@ describe('isMacOS', () => {
         expect(isMacOS()).toBeFalsy();
     });
 });
+
+describe('prefersDarkMode', () => {
+    test('is true', () => {
+        window.matchMedia = jest.fn().mockReturnValue({
+            matches: true,
+        } as MediaQueryList);
+        expect(prefersDarkMode()).toBeTruthy();
+    });
+    test('is false', () => {
+        // @ts-expect-error 2790
+        delete window.matchMedia;
+        expect(prefersDarkMode()).toBeFalsy();
+
+        window.matchMedia = jest.fn().mockReturnValue({
+            matches: false,
+        } as MediaQueryList);
+        expect(prefersDarkMode()).toBeFalsy();
+    });
+});