Type: feature request

Description:
Dear Appdroid,

I absolutely love your game Sudoku World!
It has brought me hours of joy while playing and I have even designed my own sample game grids to play with.

There is one annoying thing though - I really don't like how the subgrids don't have anything around them.
Since subgrids are supposed to not contain the same image twice, it would be really helpful if there was some sort of border around them.

Thanks,
Aurelie