Add a section on user preference customization to env.example

.env.example

@@ -282,6 +282,15 @@ PROXY_FOR_OUTGOING_REQUESTS=null
 CONTENT_SECURITY_POLICY=true
 
 
+# User preferences can be enforced and/or locked.
+# This is done by setting additional environment variables with dedicated names.
+# See https://docs.2fauth.app/getting-started/config/user-preferences/
+
+# Example to enforce light theme to all users:
+# USERPREF_DEFAULT__THEME=light
+# USERPREF_LOCKED__THEME=true
+
+
 # Leave the following configuration vars as is.
 # Unless you like to tinker and know what you're doing.