Make it possible to create custom themes for Mailpile

[BjarniRunar]

It would be nice if web developers and designers could customize the look and feel of Mailpile. This will allow Mailpile to appeal to a broader audience and involve more design-minded people in the project. E-mail is such a large part of people's lives, that it is important that people feel comfortable and at ease in their mail client. It should be beautiful, as well as functional. In addition, there are certain communities with special needs that are best addressed with custom designs, whether for branding reasons, to foster a sense of community or to alleviate a disability.