An employer cannot say something about your termination that is not true. So no, they could not say that you were fired for stealing if they fired you because they did not like the color of your car. (Although it would be legal for them to fire you because they did not like the color of your car.)
However, in this case, the company is telling the truth as they see it. They think that your behavior crossed the line into gross misconduct. They are entitled to think that, and since they believe it to be true, they are entitled to say that. No, there is no legal way you can force them to change that. Whether you think it unfair or not is immaterial.
BTW, every state except Montana is an at-will state, and only a small handful of states require that you be given any reason (and even in those states they only have to give you a reason on your written request).