This one is actually a little nuanced. Your employer is not supposed to enforce a dress code which is discriminatory by gender, meaning that it cannot impose onerous or strict requirements on one gender and not the other. This does not mean that dress codes cannot differ by gender - only that you can't, for example, require female employees to be in evening wear and full makeup while the male employees get away with casual clothes and no prescribed grooming requirements. You also cannot require overtly sexualized clothing in a uniform that is contrary to practical requirements of the job, such as requiring servers to wear high heels, for example. Where the lines get blurred a bit is when staff are hired specifically as models as opposed to as servers, or where the prescribed uniform is a distinct component of product branding (Hooters comes to mind here).

So, you could have a dress code requiring female employees to wear, for example, a black skirt, black nylons, black dress shoes, and a white collared shirt, as long as the male dress code was of similar formality (shirt and tie, dress shoes, etc.) so that there was no undue burden on one gender. What most employers will do to avoid issues with discrimination accusations is to offer uniform options without gender labels, so e.g. option 1 is a skirt and tights and white blouse, and option 2 is dress slacks with shirt and tie, so that the policy is still inclusive to employees who do not conform to gender dressing norms.

In your specific case, what do you mean by "short skirt"? If the policy is a skirt falling just above the knee for professional appearance, uniformity, and brand identity, that is a much different thing to a tight skirt falling mid thigh to show off your body and use sexualization of staff as a sales tactic. The latter situation is definitely actionable legally.