fix: test bg in app container

theme.py

@@ -33,8 +33,8 @@ class Seafoam(Base):
             font_mono=font_mono,
         )
         super().set(
-            body_background_fill="url('medias/background.png') no-repeat center center / cover",
-            body_background_fill_dark="url('medias/background.png') no-repeat center center / cover",
+            body_background_fill="url('https://storage.googleapis.com/wpassets.junyiacademy.org/1/2023/11/background-scaled.jpg') no-repeat center center / cover",
+            body_background_fill_dark="url('https://storage.googleapis.com/wpassets.junyiacademy.org/1/2023/11/background-scaled.jpg') no-repeat center center / cover",
             body_text_color_dark="#333",
             background_fill_primary_dark="rgba(255, 255, 255, 1)",
             background_fill_secondary_dark="rgba(255, 255, 255, 0)",