The default out-of-box login screen is represented by the logon.jsp file located in the lib/curam/web/jsp directory of the Client Development Environment for Java™ (CDEJ). The logon.jsp file can be customized by creating a copy of the out-of-the-box file and placing this in a webclient/components/<custom>/WebContent folder, where <custom> represents the name of the custom web client component.
The section on Login Pages in the Cúram Web Client Reference Manual has guidelines on what needs to remain in place in the logon.jsp file and should be referenced for further details.