Thanks @it33 for the links. And yes, if we would show these pages to the client, we would be happy to just drink the coffee before being shown the door 
Maybe we don't understand "whitelabel". This is not about acknowledgment of copyrights and creators. This is clear and also fair to mention that. Clients indeed need to patch, upgrade and develop additions. The need is different: It is all about a large group of potential users, such as our the client, that demand to use their own identity to their members.
And this is very common in CMS but some strange reason completely neglected in about all chat apps. It baffles me that we all seem to forget business / open source lessons learned so recently. Take Wordpress: would they exist if they did not allow theming? Each WP user can choose their own colors and logos on homepage, login page, menu structure. Users extend the functionality with plugins.
My particular client does not even care about hosting themselves, preferably not even, if they can also get what Slack is offering: full export of all data, avoiding client lock-in.The only reason while they consider open source is 1) the Theming and 2) the price: the full membership fee they charge for their associations in $1 per month... there is no room for per member paying. This is a huge market: 100.000's of associations, universities and so on have the same problem and nobody is really catering for them.. strange.. ))
Technically, Wordpress solved the theming cumbersome. But with React or even better ES6, think Aurelia.io, we could elegantly give Mattermost highly flexible Themes / Fully Customised Front Ends?
Let me finish with two mockups, note the absence of the hamburger menu:
