grid() is like pack() in that it is just a geometry manager - it only specifies positions of widgets, no decoration.

To get a decoration, of could specify a Frame, with a border, at each grid location. Then put your non-square widgets inside the frame. You might be also able to specify a background picture of a grid for you top-level window; I haven't tried this in my own work. Alternatively, one could use a Canvas, draw a grid on it, then add widgets where you want.