Via this post.
"This document covers the design and some implementation details of getting WPF windows wrapped in custom chrome. Currently WPF supports standard windows, with an icon, title-text, and caption buttons, as well as borderless windows, which when combined with transparency allow arbitrary top-level shapes but the application loses system support for behaviors associated with having a standard caption area, such as maximize. There is a desire to create applications that fill a middle-ground here: the appearance and behavior of a standard window with a more complete branding experience"
Get the code here.