What is the ruling on trimming and thinning the eyebrows, without shaping them, for beautification and to remove anything that mars the appearance, especially since the questioner wears a niqab and is seen only by her husband and mahrams?

Plucking is religiously forbidden, and permissible in two cases: therapeutic necessity, and if eyebrow hair growth is harmful or disfiguring. If the hair does not reach this extent, it is not permissible to pluck from it. The fact that a woman wears a niqab or has her husband's permission does not make plucking permissible.