settings: replace darkMode with useDarkMode hook

This is a minor breaking change for users that don't have dark mode
set to the default value since the localStorage key has changed (hard-
coded in 3rd party library).

src/utils/os.test.ts

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2021 The Pybricks Authors
 
-import { isAndroid, isMacOS, isWindows, prefersDarkMode } from './os';
+import { isAndroid, isMacOS, isWindows } from './os';
 
 afterEach(() => {
     jest.resetAllMocks();
@@ -39,18 +39,3 @@ describe('isWindows', () => {
         expect(isWindows()).toBeFalsy();
     });
 });
-
-describe('prefersDarkMode', () => {
-    test('is true', () => {
-        jest.spyOn(window, 'matchMedia').mockReturnValue({
-            matches: true,
-        } as MediaQueryList);
-        expect(prefersDarkMode()).toBeTruthy();
-    });
-    test('is false', () => {
-        jest.spyOn(window, 'matchMedia').mockReturnValue({
-            matches: false,
-        } as MediaQueryList);
-        expect(prefersDarkMode()).toBeFalsy();
-    });
-});

src/utils/os.ts

@@ -29,11 +29,3 @@ export function isMacOS(): boolean {
 export function isWindows(): boolean {
     return /win/i.test(navigator.platform);
 }
-
-/**
- * Tests if the OS is set to dark mode.
- * @returns: `true` if dark mode should be preferred, otherwise `false`.
- */
-export function prefersDarkMode(): boolean {
-    return window.matchMedia('(prefers-color-scheme: dark)').matches;
-}