Write something
Every value a Board carries, with the real window below to see it in. Keys, Custom CSS, Website CSS and your search engine live in the browser itself, because those are settings rather than part of a look.
Pick one, then change anything. Nothing here is locked.
Your own photo on the new tab page or behind the toolbars. It travels inside the .board file, so keep it small if you plan to send it to someone.
Open the .board file in Chalkboard with the Boards section of the Chalk Tray, or double click it. The Firefox version is a stylesheet instead, so the same look runs in real Firefox: the file itself says where to put it.