So is it best to stick `Common::Style::DEFAULTS` on the app *instead* of here in `Common::Style`? Or do we want both? i.e. conceptually do we want three levels? :
* Shoes Global Defaults (stroke: black, fill: black)
* App global defaults (initially empty, then user can set stroke, fill etc)
* App element defaults (initially empty, then user can set wedge, angle etc)

or two levels

* App global defaults
* App element defaults

Or something else?