I'll give some advice too as a casual sudoku player. I personally think the background was completely fine for it being made 4 years ago. If you were to remake

I'll give some advice too as a casual sudoku player. I personally think the background was completely fine for it being made 4 years ago. If you were to remake it then possibly a solid color would be a better choice now. I am also turned off by the timer. I'm not sure how many players prefer timers, but maybe putting it hidden away on the top right or somewhere that isn't blinking at you every second would help users like me. Everything else seemed good to me. If there were ads I probably wouldn't play personally (sorry adblock is on) because as the last person commented I already have a paid sudoku app on the phone that is ad-free. I think it's hard to get users to go from their phone to browsers for sudoku (if you were only targetting web-only users then it's okay) because of how good the interface is on touch panels. If you were trying to get users to move from the phone I think you need some gimmick that utilizes more of the screen space or something that the web provides that mobile can't.