Don't worry about ideas and designs. Nobody is going to steal them.
Why do people say this? Remember facebook?
People don't steal ideas. Most ideas are worthless by themselves.
It is when you've executed an idea that it becomes vulnerable, as someone can take your execution and either clone it directly or create an improved version of it. Execution itself is the hard bit, no interesting idea ever becomes fact without significant discoveries and changes in the process. The gap between the idea and the implementation can be narrowed by experience, but is never small.
In any case, the solution would be to get a lawyer to write a contract that prevents the situation you're worried about. Developing any game, even a small one, is expensive. You're going to want to protect that significant investment legally at some point, it might as well be from the beginning.