Poetess Kera, only the actor can know if the action is sinful. For conscientious people there is potential for venial sins with regard to dressing through accidental scandal. (But I would not minimize venial sin because it undermines our relationship with God and requires later purification.) Since every situation is unique no standard rule will be correct, even though at least one Pope has said there is an absolute standard. Not everbody can be scandalized, but we have to be on watch for the weak or young.
Taking a legalistic approach, to contrast with the other views here, there are some teachings of Summa Theologica of St. Thomas Aquinas and the Catholic Catechism that can be used. At least the following sins could be committed directly through dress:
- immodesty (revealing too much, pertaining to the sixth and ninth commandments)
- disrespect for God expressed by inappropriate attire in Church, etc.
- disrespect for others, causing them embarassment, by inappropriate attire
- disrespect for authority (political, religious, or elders) by inappropriate attire
- (venial only) false pride, i.e., ostentation or pride of appearence for one’s beauty or wealth
- scandal (fifth commandment)
Saint Thomas Aquinas details scandal (setting a bad example or tempting another to sin) in the following way:
Intention to lead other to mortal sin = mortal sin
Intention to lead other to venial sin by mortal sin = mortal sin
Accidentally through a mortal sin = mortal sin
Intention to lead other to venial sin by venial sin = venial sin
Accidentally through a venial sin = venial sin
Accidentally through a non-sin with appearence of evil = venial sin