Is working as an accountant for a jewelry company that manufactures crosses impermissible (haram)? And does the same ruling apply to the workers who make them with their own hands?

It is permissible for a Muslim to work for a non-Muslim if the work is permissible. If the company specializes only in making crosses, then it is not permissible to work for them, as this constitutes aiding in sin. However, if they have other permissible works, then it is permissible to work in the permissible areas, but it is not lawful to do accounting work related to crosses. Muslim workers who make crosses are even more prohibited from doing so, as the Prophet (peace and blessings be upon him) used to break and invalidate anything with crosses in his home.