Praise be to Allah.
It says in Fataawa al-Shaykh Muhammad ibn Saalih al-‘Uthaymeen:
Plucking of women’s eyebrows is not permissible, it is the namas for which the Messenger of Allaah (peace and blessings of Allaah be upon him) cursed the one who does it. The naamisah is the one who does it for someone else and the mutanammisah is the one who asks someone else to do it for her (as mentioned in the hadeeth). The same applies if a woman does it for herself. It is haraam and is not permitted.
Allaah is Wise in what He decrees for His slaves. Some people are beautiful in appearance and some are not; the entire matter is in the hand of Allaah. It is essential to be patient and seek reward with Allaah, and not to transgress His sacred limits for the sake of one's own whims and desires.
What I think is that you should not remove anything from the eyebrows at all, unless there is hair outside of the line of the eyebrows, such as if there is a mole from which a hair is growing. This may be removed because in this case it is removing a kind of fault or deformity, not in order to look beautiful. And Allaah knows best.
See Fataawa Manaar al-Islam by Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him), vol. 3, p. 832.
Shaykh Ibn Jibreen said: “It is not permissible to cut the hair of the eyebrows, or to shave it, reduce it or pluck it, even with the husband’s approval. This is not the matter of beauty, rather it is altering the creation of Allaah Who is the Best of creators. A warning has been narrated against doing that and the one who does it is cursed; this implies that it is forbidden.