feat: add system mode to the theme selector (#7853)

Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

setupTests.ts

@@ -11,6 +11,20 @@ require("fake-indexeddb/auto");
 
 polyfill();
 
+Object.defineProperty(window, "matchMedia", {
+  writable: true,
+  value: vi.fn().mockImplementation((query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(), // deprecated
+    removeListener: vi.fn(), // deprecated
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+});
+
 vi.mock("nanoid", () => {
   return {
     nanoid: vi.fn(() => "test-id"),