Add tests for event theme API functionality

Introduces comprehensive tests for creating, listing, updating, and retrieving event themes. Includes scenarios with valid data, invalid data, and pagination to ensure robust API behavior. Adds necessary fixtures to support the new test cases.

backend/tests/conftest.py

@@ -1,6 +1,7 @@
 # tests/conftest.py
 import uuid
 from datetime import datetime, timezone
+from typing import Dict
 
 import pytest
 
@@ -326,3 +327,23 @@ def gift_category_fixture(db_session, mock_user, event_fixture):
     db_session.add(gift_category)
     db_session.commit()
     return gift_category
+
+
+@pytest.fixture
+def theme_data() -> Dict:
+    return {
+        "name": "Animal Safari",
+        "description": "A wild and fun theme with safari animals",
+        "preview_image_url": "https://example.com/safari.jpg",
+        "color_palette": {
+            "primary": "#f4a261",
+            "secondary": "#2a9d8f",
+            "background": "#ffffff",
+            "text": "#264653"
+        },
+        "fonts": {
+            "header": "Safari Display",
+            "body": "Nunito Sans"
+        }
+    }
+