diff --git a/docs/security.md b/docs/security.md
index 545fbabf..580174e1 100644
--- a/docs/security.md
+++ b/docs/security.md
@@ -22,6 +22,7 @@ On upgrade from older databases, existing plaintext secrets in SQLite are **migr
 
 Automated tests use an **in-memory** secrets backend (see `test/flutter_test_config.dart`) so CI does not require a desktop keyring.
 
+
 ## Archive install limits (extensions and updates)
 
 Marketplace downloads, local extension sideload (`.zip` / `.qext`), and in-app updater extraction use `SafeZipExtractor` (`lib/core/security/safe_zip_extractor.dart`) with shared default limits:
@@ -35,3 +36,4 @@ Marketplace downloads, local extension sideload (`.zip` / `.qext`), and in-app u
 | Max compression ratio (uncompressed ÷ compressed) | 100:1 |
 
 Archives exceeding these bounds fail closed before files are written to disk. Path traversal checks remain in `archive_path_guard.dart`.
+
diff --git a/lib/core/theme/theme_remote_install_service.dart b/lib/core/theme/theme_remote_install_service.dart
index fa958c89..7b50bde6 100644
--- a/lib/core/theme/theme_remote_install_service.dart
+++ b/lib/core/theme/theme_remote_install_service.dart
@@ -75,6 +75,12 @@ class ThemeRemoteInstallService {
     final expectedChecksum = _normalizeSha256(
       sha256Checksum ?? uri.queryParameters['sha256'],
     );
+    if (expectedChecksum == null) {
+      return const ThemeDefinitionImportFailure(
+        'SHA256 checksum is required for remote theme install. '
+        'Add ?sha256= to the URL or pass sha256Checksum.',
+      );
+    }
 
     File? tempFile;
     try {
@@ -92,7 +98,7 @@ class ThemeRemoteInstallService {
       }
 
       final actualChecksum = sha256.convert(utf8.encode(body)).toString();
-      if (expectedChecksum != null && expectedChecksum != actualChecksum) {
+      if (expectedChecksum != actualChecksum) {
         return const ThemeDefinitionImportFailure(
           'Checksum mismatch. Theme was not installed.',
         );
diff --git a/test/core/theme/theme_remote_install_service_test.dart b/test/core/theme/theme_remote_install_service_test.dart
index ad80b0cb..8f10ac5a 100644
--- a/test/core/theme/theme_remote_install_service_test.dart
+++ b/test/core/theme/theme_remote_install_service_test.dart
@@ -1,5 +1,7 @@
+import 'dart:convert';
 import 'dart:io';
 
+import 'package:crypto/crypto.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:path/path.dart' as p;
 import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
@@ -26,6 +28,8 @@ Future<String> _fixtureAssetLoader(String assetPath) async {
   return File(p.join('test/fixtures/themes', fileName)).readAsString();
 }
 
+String _sha256Hex(String body) => sha256.convert(utf8.encode(body)).toString();
+
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
@@ -100,6 +104,7 @@ void main() {
 
       final result = await service.installFromUrl(
         'https://cdn.example.com/themes/querya_custom_dark.json',
+        sha256Checksum: _sha256Hex(raw),
       );
 
       expect(result, isA<ThemeDefinitionImportSuccess>());
@@ -136,18 +141,37 @@ void main() {
       expect(await ExtensionPaths.mockExtensionsDirectory!.list().length, 0);
     });
 
+    test('rejects install when SHA256 checksum is missing', () async {
+      final service = ThemeRemoteInstallService(
+        registry,
+        allowLocalhostInDebug: false,
+      );
+
+      final result = await service.installFromUrl(
+        'https://cdn.example.com/themes/querya_custom_dark.json',
+      );
+
+      expect(result, isA<ThemeDefinitionImportFailure>());
+      expect(
+        (result as ThemeDefinitionImportFailure).message,
+        contains('SHA256 checksum is required'),
+      );
+    });
+
     test('rejects invalid JSON without writing to themes folder', () async {
+      const brokenBody = '{ not valid json';
       final service = ThemeRemoteInstallService(
         registry,
         allowLocalhostInDebug: false,
         httpGet: (_) async => const RemoteThemeHttpResponse(
           statusCode: 200,
-          body: '{ not valid json',
+          body: brokenBody,
         ),
       );
 
       final result = await service.installFromUrl(
         'https://cdn.example.com/themes/broken.json',
+        sha256Checksum: _sha256Hex(brokenBody),
       );
 
       expect(result, isA<ThemeDefinitionImportFailure>());
@@ -219,6 +243,7 @@ void main() {
 
       final result = await service.installFromUrl(
         'https://cdn.example.com/themes/querya_custom_dark.json',
+        sha256Checksum: _sha256Hex(raw),
       );
 
       expect(result, isA<ThemeDefinitionImportSuccess>());
@@ -235,6 +260,7 @@ void main() {
 
       final result = await controller.importRegistryThemeFromUrl(
         'https://cdn.example.com/themes/querya_custom_dark.json',
+        sha256Checksum: _sha256Hex(raw),
         remoteInstallService: ThemeRemoteInstallService(
           registry,
           allowLocalhostInDebug: false,