What is the criterion for not imitating disbelievers in matters prevalent among Muslims, and does this apply at the level of Islamic nations or a single nation, and do leggings, accessories, and hairstyles currently widespread among girls inside the home count as imitating disbelieving women?

The criterion for forbidden imitation of disbelievers is imitating them in what is specific to them, such that anyone who sees the person would assume them to be one of them. As for what is not specific to them, and the doer would not be assumed to be one of them, it does not fall under dispraised imitation. The number of people who do it is not a condition. As for women's clothing, adornment tools, and hairstyles, if they are free from being specific to disbelieving and immoral women, and from being a symbol for them, then there is no harm in using them, whether few or many Muslim women use them. Otherwise, it is not permissible. There is no difference in dispraised imitation between what is done outside the home and what is done inside it. It must be noted that wearing "stretch" clothing and the like without anything to cover it is not permissible except in front of one's husband.