Add is_active field to EventTheme model and test fixture.

The `is_active` field was introduced to the `EventTheme` model to indicate whether a theme is active. The corresponding test fixture in `conftest.py` was updated to include this new field, ensuring consistency in tests. This change enhances flexibility for managing event themes.

backend/tests/conftest.py

@@ -197,6 +197,7 @@ def event_theme_fixture(db_session):
         name="Animal Theme",
         description="An animal-themed design for events.",
         preview_image_url="https://example.com/preview/animal_theme.jpg",
+        is_active=True,
         color_palette={
             "primary": "#ff5722",
             "secondary": "#4caf50",