Fix icon on empty notification panel (#10817)

* Use custom properties for the empty panel icons on FilePanel and NotificationPanel

* Create E2E test for the notification panel

cypress/e2e/right-panel/notification-panel.spec.ts

@@ -0,0 +1,52 @@
+/*
+Copyright 2023 Suguru Hirahara
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/// <reference types="cypress" />
+
+import { HomeserverInstance } from "../../plugins/utils/homeserver";
+
+const ROOM_NAME = "Test room";
+const NAME = "Alice";
+
+describe("NotificationPanel", () => {
+    let homeserver: HomeserverInstance;
+
+    beforeEach(() => {
+        cy.startHomeserver("default").then((data) => {
+            homeserver = data;
+            cy.initTestUser(homeserver, NAME).then(() => {
+                cy.createRoom({ name: ROOM_NAME });
+            });
+        });
+    });
+
+    afterEach(() => {
+        cy.stopHomeserver(homeserver);
+    });
+
+    it("should render empty state", () => {
+        cy.viewRoomByName(ROOM_NAME);
+        cy.findByRole("button", { name: "Notifications" }).click();
+
+        // Wait until the information about the empty state is rendered
+        cy.get(".mx_NotificationPanel_empty").should("exist");
+
+        // Take a snapshot of RightPanel
+        cy.get(".mx_RightPanel").percySnapshotElement("Notification Panel - empty", {
+            widths: [264], // Emulate the UI. The value is based on minWidth specified on MainSplit.tsx
+        });
+    });
+});