Enable theming of the mail template header

First off: I could not test this because I do not have a mail server set up at the moment. Can you point me to the docs how to do that? Secondly, please review @karlitschek @MTRichards @DeepDiver1975 @owncloud/designers 

* remove off-white background color to better blend into mail clients
* compress logo used in mail notifications
* make mail notification header color themable