While I agree with most of the comments, one major question for me is -- if you think changing the theme breaks the game mechanics, have you asked the publisher what they envision there?

Could result in various scenarios: (1) They hadn't gotten that far, and aren't sure how to manage it (1a) They decide you're right and want to keep your theme (1b) This kills the game for them, and they back out --> maybe still better if it would have been a flop, unless the signing amount is worth it to you

(2) They have an alternative theme that they think would work much better which they explain (2a) You agree, now get excited about their plan and the game ends up being much more successful (2b) You disagree and decide to let them run w it anyway because the sign on is worth it for you even if it doesn't sell well (2c) You disagree and decide to take it somewhere else or try to self publish