You can configure Identity Server in two ways to use a custom login page. You can use properties or you can modify the main.jsp file. Select the method depending upon your modifications.
Use properties if you created your custom page from Access Manager 3.2 or later login.jsp page. See Using Properties to Specify the Login Page.
If you created your custom page from the nidp_legacy.jsp file, you cannot use properties to specify the main custom page for authentication. You must modify the main.jsp file. See Adding Logic to the main.jsp File.