I think you can create something on any theme you like. It's not bad decision, It's ok to do so! In game jam you have to be inspired about what you are doing, I understand you here. But I cannot change the theme now, so just do something on any other theme you prefer. You can make, for example, a space game where you have ship repairing - just use wrench model or sprite and make that you have to find this wrench to repair your ship in night land.
While making your game just find a place to include things from the theme. Hope It helps :)