diff --git a/frontend/src/App.test.tsx b/frontend/src/App.test.tsx
index 3fc9c18..0580ba9 100644
--- a/frontend/src/App.test.tsx
+++ b/frontend/src/App.test.tsx
@@ -21,18 +21,23 @@ Object.defineProperty(window, 'matchMedia', {
 });
 
 // Mock fetch
-global.fetch = jest.fn((url: string) => {
+global.fetch = jest.fn((input: RequestInfo | URL, init?: RequestInit) => {
+  const url = typeof input === 'string' ? input : input instanceof URL ? input.toString() : input.url;
   if (url.includes('/api/files') && !url.includes('/api/files/')) {
     return Promise.resolve({
       ok: true,
+      status: 200,
+      headers: new Headers(),
       json: () => Promise.resolve({ files: ['test.md'] }),
-    });
+    } as Response);
   }
   return Promise.resolve({
     ok: true,
+    status: 200,
+    headers: new Headers(),
     json: () => Promise.resolve({}),
-  });
-}) as jest.Mock;
+  } as Response);
+}) as jest.MockedFunction<typeof fetch>;
 
 describe('App', () => {
   beforeEach(() => {
@@ -54,22 +59,32 @@ describe('App', () => {
   });
 
   test('opens file when clicked', async () => {
-    // Mock the file content fetch
-    fetch.mockImplementationOnce((url: string) => {
+    // Mock the file content fetch with a custom mock
+    const mockFetch = jest.fn((input: RequestInfo | URL, init?: RequestInit) => {
+      const url = typeof input === 'string' ? input : input instanceof URL ? input.toString() : input.url;
       if (url.includes('/api/files')) {
         return Promise.resolve({
           ok: true,
+          status: 200,
+          headers: new Headers(),
           json: () => Promise.resolve({ files: ['test.md'] }),
-        });
+        } as Response);
       }
       if (url.includes('test.md')) {
         return Promise.resolve({
           ok: true,
+          status: 200,
+          headers: new Headers(),
           json: () => Promise.resolve({ content: '# Test Content' }),
-        });
+        } as Response);
       }
-      return Promise.resolve({ ok: true });
+      return Promise.resolve({
+        ok: true,
+        status: 200,
+        headers: new Headers(),
+      } as Response);
     });
+    global.fetch = mockFetch;
 
     render(<App />);
     
@@ -89,15 +104,23 @@ describe('App', () => {
 
   test('creates new file', async () => {
     // Mock the create file endpoint
-    fetch.mockImplementationOnce((url: string) => {
-      if (url.includes('/api/files') && fetch.mock.calls.length === 1) {
+    const mockFetch = jest.fn((input: RequestInfo | URL, init?: RequestInit) => {
+      const url = typeof input === 'string' ? input : input instanceof URL ? input.toString() : input.url;
+      if (url.includes('/api/files') && !init?.method) {
         return Promise.resolve({
           ok: true,
+          status: 200,
+          headers: new Headers(),
           json: () => Promise.resolve({ files: [] }),
-        });
+        } as Response);
       }
-      return Promise.resolve({ ok: true });
+      return Promise.resolve({
+        ok: true,
+        status: 200,
+        headers: new Headers(),
+      } as Response);
     });
+    global.fetch = mockFetch;
 
     render(<App />);
     
@@ -117,7 +140,7 @@ describe('App', () => {
 
     // Verify fetch was called with POST
     await waitFor(() => {
-      expect(fetch).toHaveBeenCalledWith('/api/files', {
+      expect(mockFetch).toHaveBeenCalledWith('/api/files', {
         method: 'POST',
         headers: {
           'Content-Type': 'application/json',
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index f0bff99..644d26b 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,7 +1,7 @@
 import React, { useState, useEffect } from 'react';
 import ReactMarkdown from 'react-markdown';
-import { Box, Typography, TextField, Button, List, ListItem, ListItemText, IconButton, Divider, AppBar, Toolbar, Drawer, CssBaseline, useTheme, ThemeProvider, createTheme, Select, MenuItem, FormControl, InputLabel } from '@mui/material';
-import { Delete, Edit, Add, Save, Menu as MenuIcon } from '@mui/icons-material';
+import { Box, Typography, TextField, Button, List, ListItem, ListItemText, IconButton, Divider, AppBar, Toolbar, Drawer, CssBaseline, ThemeProvider, createTheme, Select, MenuItem, FormControl, InputLabel, SelectChangeEvent } from '@mui/material';
+import { Delete, Add, Save, Menu as MenuIcon } from '@mui/icons-material';
 
 interface FileItem {
   filename: string;
@@ -116,7 +116,7 @@ const App: React.FC = () => {
     },
   });
 
-  const handleThemeChange = (event: React.ChangeEvent<{ value: unknown }>) => {
+  const handleThemeChange = (event: SelectChangeEvent<'light' | 'dark' | 'system'>) => {
     setThemeMode(event.target.value as 'light' | 'dark' | 'system');
   };