To create your own theme for Microsoft Exchange 2010 you can copy the files from one of the existing themes and customize them to you liking. And easy way to do it is to copy the theme that most resembles your desired outcome, end then simply customize the header image, and preview thumbnail.
When you are done simply create a new folder in the themes directory and copy your customized theme.
The theme file can be found in here “…\V14\Client Access\OWA\<version number>\themes”
E.g. “C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa\14.1.218.13\themes”
You can also customize the sign-in and sign-out pages which can be found here:
“\V14\Client Access\OWA\<version number>\themes\resources”
More info here: http://technet.microsoft.com/en-us/library/ee633483.aspx