Can I please have references for whether modifying, setting or doing anything with the eye brows for the sole purpose of looking good/better is permissible for our Muslim men/women or not?
(This answer is based on view and fatwa of current Maraji)
According to different hadith about beauty of woman and duties of woman for her husband totally (without considering some specific hadith without a comprehensive view) it is concluded in short:
Generally any kind of beauty of women if is done for non-Mahram men or used in public it is Haram. But if it is only for Husband and non-Mahram men do not see it there is no problem. The main reason of such beauty is that non-Mahram see it. But if a woman can make a beauty in such a manner that non-Mahram men do not see it there is no problem.
Also trimming of eye brow slightly that is not considered beauty and is only for having a regular look (not as beauty) has no problem to be seen in public.
There are different hadith from Messenger SAWW and Ahl ul Bayt that shows the woman has the duty of beauty for her husband and beauty is harm only in public and view of non-mahram men.
Please note making any beauty including any injury to body is a different topic. This is about beauty without any harm to body.
In this article this topic is researched in detail and with different hadith from Messenger SAWW and Ahl ul Bayt like Imam Sadiq a.s. and Imam Baqir a.s.
You can study all of it using the google translate but the summary is what I wrote above.
I had written an answer to a similar question on another site, which I am copying (with slight modification) below:
Plucking of the eyebrows goes against the sunnah of the prophet and should not be done; much like getting a tattoo or lengthening the hair, it is considered a form of changing what Allah has created. There are a number of ahadith wherein the prophet has cursed those who attempt to beautify themselves with such actions, or to perform them on others. Here is one rather clear example narrated from Abdullah (Sahih Bukhari 5948):
Allah has cursed those women who practise tattooing and those who get themselves tattooed, and those who remove their face hairs, and those who create a space between their teeth artificially to look beautiful, and such women as change the features created by Allah. Why then should I not curse those whom the Prophet has cursed? And that is in Allah's Book. i.e. His Saying: 'And what the Apostle gives you take it and what he forbids you abstain (from it).' (Quran 59:7)
Whether "...those who remove their face hairs..." refers to all hairs of the face or just the eyebrows rests on the translation of the Arabic word "al-naamisah". Although it can be literally interpreted to mean a woman who removes any of her facial hairs at all (rather than the eyebrows in particular), the common opinion is that al-naamisah refers only to women who pluck and shape their eyebrows for beautification and that plucking other facial hairs (such as, for women, those in the beard or mustache area) is acceptable.