Microsoft has provided some basic details on how to customize the Outlook Web App logon page (OWA) in Exchange 2013. I’ve taken things a bit further and modified logon.aspx and logon.css to customize the OWA logon page with a PowerShell script.
It is important to note that at this time the script does not back up any of the files that it modifies. Be warned, you should backup logon.aspx and logon.css. As of Exchange 2013 SP1 CU6 they are located in the following directories:
Logon.aspx: C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth
Logon.css: C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth\15.0.995\themes\resources