✨(back) allow theme customnization using a configuration file

We want to customize the theme by using a configuration file. This
configuration file path can be defined using the settings
THEME_CUSTOMIZATION_FILE_PATH. If this file does not exists or is an
invalid json, an empty json object will be added in the config endpoint.

src/backend/impress/settings.py

@@ -440,6 +440,18 @@ class Base(Configuration):
         None, environ_name="FRONTEND_CSS_URL", environ_prefix=None
     )
 
+    THEME_CUSTOMIZATION_FILE_PATH = values.Value(
+        os.path.join(BASE_DIR, "impress/configuration/theme/default.json"),
+        environ_name="THEME_CUSTOMIZATION_FILE_PATH",
+        environ_prefix=None,
+    )
+
+    THEME_CUSTOMIZATION_CACHE_TIMEOUT = values.Value(
+        60 * 60 * 24,
+        environ_name="THEME_CUSTOMIZATION_CACHE_TIMEOUT",
+        environ_prefix=None,
+    )
+
     # Posthog
     POSTHOG_KEY = values.DictValue(
         None, environ_name="POSTHOG_KEY", environ_prefix=None