If you want to show any custom message to user. You need NOT to change the core code and not to override the template, Content App will help to configure your scenario.
You just need to install the Content App from App-Store and create its app instance.
Follow the below steps, to configure details in Content App:
Apply onto Page : Select any predefined page to display the content.
Position : Select appropriate position of content like - Prefix to Existing Content,** Suffix to Existing Content** or Replace Existing Content.
Display content from : Either add your custom content in to app itself or you can provide link of existing Joomla Article.