Shabbos 64b - Not to be Degraded in the Eyes of Her Husband

The gemara says that chazal wanted to forbid a woman from wearing any kind of tachshit on shabbos out of fear that she may come to carry it, but permitted two specific types of head coverings שלא תתגנה על בעלה. The gemara continues that we find a similar concept. Chazal wanted to darshen from the pasuk of והדוה בנדתה which implies she must maintain herself in a state of nidda, that when a woman is a nidda she is forbidden to put on make up, jewlery or other types of adornments. However, Rabbi Akiva rejected this drasha כדי שלא תתגנה על בעלה which may lead to him divorcing her, and therefore darshened the pasuk to simply say that she remains a nidda until going to the mikvah. 

Firstly, it seems strange how R. Akiva could reject the drasha based on the concern that it will be bad for shalom bayis and lead to divorce. It seems as if the zekainim harishonim were not merely instituting an issur d'rabonon but rather darshening a pasuk. How can R. Akiva reject this drasha on the basis of it detracting from shalom bayis? We see from here that R. Akiva could not accept their interpretation of the pasuk to be an accurate drasha because he felt that there is no way the Torah would forbid something that will lead to her being מתגנה על בעלה which can ultimately end in divorce. Based on his sevara he insists that the Torah mean something else entirely.

In explaining the heter for a woman to wear these head pieces on shabbos to prevent her from being מתגנה על בעלה, Rashi writes התירו לה קצת קישוטים הנאים. At first it would seem that in the context of nidda also the sevara should only permit her to war קצת קישוטים הנאים, some nice adornments, only enough that she not be מתגנה על בעלה. However, from the fact that Rashi only writes this in the context of shabbos prior to citing the Braisa by nidah, it implies that when R. Akiva rejects the drasha by niddah כדי שלא תתגנה על בעלה, he rejects it entirely. Therefore, a woman who is a nidah would be allowed to dress however nice and fancy she chooses, and is not required to hold back any form of קישוט. The rationale for the distinction is simple. By shabbos where the Rabbonon are concerned about carrying, they would only permit the bare minimum necessary to prevent her from being מתגנה על בעלה. But for a Nidah, once R. Akvia rejects the drasha there is no basis for any limitation. This seems to be the approach of the Rambam who writes (End of perek 11 in Issurei Biah) - מותר לאשה להתקשט בימי נדתה כדי שלא תתגנה על בעלה. The Rambam sounds like we permit everything without any hesitation.

However, the Shulchan Aruch (Y.D. 195:9) in recording this halacha writes:

בקושי התירו לה להתקשט בימי נדתה אלא עדי שלא תתגנה על בעלה. The language of בקושי and the language of אלא imply that there actually is an issur. It sounds like the Shulchan Aruch is understanding that we would only permit what is necessary. It is true that one can read the Shulchan Aruch to simply be saying that they would not have permitted it if not for שלא תתגנה על בעלה, but now that they permitted it, everything is mutar as the Rambam writes. But, if that is the intent of the Shulchan Aruch, all he is really saying is a historical fact of what would have been. Why would this be paskened as a halacha in Shulchan Aruch? Perhaps this question compels the Gr"a (13) to cite a braisa in Avos D'rabi Nosson:

כל המנבלת עצמה בימי נדתה רוח חכמים נוחה הימנו, וכל המתקשטת עצמה בימי נדתה אין רוח חכמים נוחה הימנו. The braisa says that it is proper for a woman to make herself look unattractive during her niddah time. The Gr"a comments that this braisa is not in accordance with the z'kainim harishonim who R. Akiva disagreed with because according to them it would be a real issur, not just אין רוח חכמים נוחה הימנו. Therefore, it must be even according to R. Akivah. Based on the Gr"a, although chazal did not forbid ANY forms of tachshit for a nidah and she is allowed to dress herself up as nicely as she wants, on a personal level she should strive to not make herself look attractive during her time of being a niddah. This is the message that the Shulchan Aruch is trying to say. R. Akiva rejected the notion that chazal would impose an issur, but he didn't reject the perspective that while a woman is a niddah she should not be מתקשטת. Based on this, although chazal couldn't forbid a niddah from being מתקשטת, it is still recommended on a personal level to abstain from doing so.
This is against some of the commentaries on avos d'rabi nosson who assume that the braisa cited at the beginning of perek 2 doesn't follow the opinion of R. Akiva. Based on this, we would follow R. Akiva so that it isn't even commendable to abstain from קשוט during her niddah time. On the other hand, the exact opposite is also a possibility. The Toras Hashlamim (also cited in sidrei tahara) suggests that only R. Akiva is very worried about her being מתגנה על בעלה because he holds (9th perek of gittin) that grounds for divorce could be as simple as finding another woman he likes better. Since R. Akiva is so permissive about divorce, he has to take special measures to prevent it. Therefore, we who don't pasken like R. Akiva in being so lax about divorce, would not be so sensitive to her being מתגנה על בעלה and would hold that it is forbidden for her to be מתקשט while she is a niddah.