This has been commented. In terms of design tools, more than single stroke drawing-mode it would be just adding a stroke parameter beside the Fill parameter.
That stroke parameter can be from very simple first, just colour, and desirably also width, to complex, adding more choices like, corner type, discontinued with line and gaps, brush type and so on.
Also a toggle button to switch fill/stroke… the regular design tool. And it shoul be for any machine, of course, not just for the iCraft 
We have a Topic with the wishlist for Idea Studio, I suggest an @admin merges this suggestion there.