test(docs-infra): fix warnings in `ThemeToggleComponent` unit tests. (#42259)

due to unknown `<mat-icon>` element

This commit fixes some warnings in the unit tests of the
`ThemeToggleComponent`, which were caused by the following:

- The `<mat-icon>` element used in `ToggleThemeComponent`'s template was
  not declared in tests.
- The `dark-theme.css` and `light-theme.css` files requested by
  `ToggleThemeComponent` were not available.

PR Close #42259

aio/angular.json

@@ -172,7 +172,17 @@
               "src/google385281288605d160.html"
             ],
             "styles": [
-              "src/styles/main.scss"
+              "src/styles/main.scss",
+              {
+                "inject": false,
+                "input": "src/styles/custom-themes/dark-theme.scss",
+                "bundleName": "dark-theme"
+              },
+              {
+                "inject": false,
+                "input": "src/styles/custom-themes/light-theme.scss",
+                "bundleName": "light-theme"
+              }
             ],
             "scripts": []
           }

aio/src/app/shared/theme-picker/theme-toggle.component.spec.ts

@@ -1,4 +1,5 @@
 import { ComponentFixture, TestBed } from '@angular/core/testing';
+import { MatIconModule } from '@angular/material/icon';
 import { By } from '@angular/platform-browser';
 import { ThemeStorage, ThemeToggleComponent } from './theme-toggle.component';
 
@@ -45,7 +46,7 @@ describe('ThemeToggleComponent', () => {
   beforeEach(() => {
     TestBed.configureTestingModule({
       declarations: [ ThemeToggleComponent ],
-      providers: [ { provide: ThemeStorage, useClass: FakeThemeStorage } ],
+      imports: [ MatIconModule ],
     });
 
     fixture = TestBed.createComponent(ThemeToggleComponent);