Customize OWA logon page in Exchange 2013Posted: June 8, 2015
The Outlook Web App sign-in, language selection, and error pages are created based on graphics and .css files in the themes resources folder in the Client Access Server.
Any modifications to those pages will be seen by all users.
Here you have an example about what you can do.
Browse to the Following Location in your Client Access Server
and modify the following files to customize Web form.