Add CRUD operations and tests for EventTheme management

Introduced CRUD implementation, schema definitions, and tests for EventTheme. This allows creation, retrieval, updating, deletion, and querying of active event themes. Comprehensive tests ensure functionality works as intended, including edge cases for nonexistent themes.

backend/app/schemas/event_theme.py

@@ -0,0 +1,31 @@
+from typing import Dict, Optional
+from pydantic import BaseModel
+
+
+class EventThemeBase(BaseModel):
+    name: str
+    description: Optional[str] = None
+    preview_image_url: Optional[str] = None
+    color_palette: Dict[str, str]
+    fonts: Dict[str, str]
+
+
+class EventThemeCreate(EventThemeBase):
+    pass
+
+
+class EventThemeUpdate(EventThemeBase):
+    name: Optional[str] = None
+    color_palette: Optional[Dict[str, str]] = None
+    fonts: Optional[Dict[str, str]] = None
+
+
+class EventThemeInDBBase(EventThemeBase):
+    id: str
+
+    class Config:
+        from_attributes = True
+
+
+class EventTheme(EventThemeInDBBase):
+    pass