Hello,
Congratulations for the excellent work. I like very much your Sudoku game design. Among a lot of Sudoku games available, I prefer yours by far. However, there are only two things that could greatly improve its user interface. I'll start with the most important one:
1. Increase thefont size of the numbers and candidates. The number should sort of fill up the cell in which it is displayed. For the candidates, you can also increase their size and put the candidate 5 in the center of the cell. No need to shift it from the center to give room for the final number. When the final number is entered, all the candidates of the given cell should be erased. If by mistake one enters the wrong cell's number and erases the candidates, they can still undo their last entry.
2. Remove the left and right margins and make the size of the window fixed, not resizable.
Thanks again for the good work.
Geo