This is really good!
I particularly appreciate the extra level design mileage you squeezed out of something as seemingly-minor as having a little in-game drawn border that's separate from the actual window border.
I did find myself wishing (several times) that I could somehow resume drawing my one line after I'd stopped pressing the mouse button, though I suppose that could get tricky. It'd almost at least feel better if the pencil just "snapped" when you stopped using it, to make it utterly clear from the first use that you can't continue your line if you stop holding the mouse button down.
Anyway, great job! And kudos to the art/sound person as well! Art/music/sound is all very endearing. Well done!