The Outlook Web App are created based on graphics and .css files in the themes resources folder use for sign-in, language selection, and error pages. These Outlook Web App uses only one set of language selection, sign-in and error pages for all themes. If the changes made is any pages then those pages will be seen by all users. You can realize the theme resources folder within the Exchange installation directory at V15\FrontEnd\HttpProxy\owa\auth\version\themes\resources. For change in default colors you will need a text editor , and a graphics editor needed to change the images. If you need to match a selected color and you cannot realize a match for it at Color Table, you’ll be able to use a picture writing tool to sample a color and confirm its hypertext markup language RGB price.
If you have multiple servers those supporting Outlook Web App and if you just want to be use them by the same sign-in, language, and error pages, then you must copy the modified files to each server. And you must also create a back-up copy of your customized files. If you want to be reinstall Exchange, then all files in the folders will be overwritten. You will have to copy your files to the appropriate folder, when the re-installation get complete.
Click here for Microsoft Outlook Web App themes.