I see. You know that if you use a custom theming on the server, it will be reflected in the clients? Of course this has it limitation, depending on what kind of company branding you’re looking at.
I don’t think it is worth the hassle, tbh. Either you need to do it without push (which really limit the apps) or you need to implement your own push proxy solution. In both cases you also need to make sure to keep up with the official development.
Also there is branding support as part of an enterprise subscription, which might be worth looking into.