To enable maintenance messages in the JSP-Loginapp, configure the Maintenance Message Settings in Loginapp >> Miscellaneous Settings. It should use the same persister plugin for maintenance messages as configured in the Adminapp.
Styling of maintenance messages
To use the full feature set of the HTML styling in maintenance messages, it is necessary to relax the content security policy (CSP). By adding a "style-src" policy statement to the CSP, the CSP will permit browsers to render unsafe HTML.
For more information on content security policy see Content Security Policy for the Loginapp REST UI.