I guess this is just early days but it crashed on me 2 steps into playing a puzzle. It also generated a puzzle with an obvious error, a 3 inline with an open box which could only have a 3 in it.
Overall it is very unattractive. It needs to be tidied up and simplified.
Use some white space, don't crowd things and offer contrast in line thickness and characters.
Needs to make viewing selections straight forward as well.
It is not like there are that many options to Sudoku:
View : w/wo hints, w/wo eliminations, color of answers, font, number of squares, background
Difficulty: Easy, Medium, Hard, Einstein
Feedback: Show errors/Don't show