The Necronia Identity Service uses Keycloak. Out of the box, the UI provided by Keycloak is quite different from the rest of Necronia. But Keycloak allows a customer to provide alternative themes to replace the default Keycloak themes. We use this mechanism to provide an Necronia theme.
This project currently uses the approach of a custom login page (option #3).
Since Keycloak supports Freemarker, a template file is provided along with css styles and images like those documented to look like the approved designs
The project also includes improvements to the translated messages for the languages supported by Necronia.