I am not married and I tread lightly on this particular issue.
The BF and I both come from a place where our last relationship was both very serious (he was married, I was long term with and engaged to mine) and also controlling.
My ex made me feel very badly about myself and that in turn affected how I dress. My BF's ex was very critical about clothing and tried to change pretty much everything about his appearance.
When we first got together he was both very concerned with what I thought about his appearance, but also at the same time very resistant to any criticism or suggestion for change. Although I certainly wasn't thrilled by how he dressed or groomed himself it wasn't a big deal and I understood that was a sore spot with him.
As time progressed I've found that many of the things that I didn't much care for weren't things he wasn't fervent about, but rather things women (particularly his ex) had encouraged. He met his ex straight after leaving the military and while on the job in uniform (he still wears a uniform to work, but his previous position had strict conservative grooming standards) and she had expectations that his work look mimicked how he dressed in the rest of his life. She was also older than him and wanted him to fit in with her friends and preferred a more conservative/preppy style (wanted him clean shaven, waxed back and chest, no cargo pants, polo shirts, pastel and collared shirts, gelled hair). Honestly, his style is much more a mix of rugged casual and rocker (based on his personality, items he admires, and older photos of him). The jeans and khakis and polos are the wrong style on him for his body type and the look really ages him about 10-15 yrs . . . he looks like a soccer dad and doesn't seem as comfortable in his skin. He still has personal style, but he seems like he has given up on it for himself.
He also wears a lot of cargo pants and message style graphic ts in black-- almost as an overreaction to her influence. Unfortunately it is almost impossible to find message ts in tall sizes, and they can be a little juvenile in contrast to "soccer dad." Black is not my favorite color on him. He also tends to wear his cargos in his "military ACU" size, which means they aren't long enough because he doesn't tuck them into tall boots! Of course, my pet peeve is PPL and fit in general.
Over time the BF has realized that I am not much of a nag and don't want to change who he is. I'm pretty good at dressing men within their specifications (I did all the shopping for my dad and brother and grandfather growing up, and they are very picky). Still, I tend to let the topic of clothes I don't like come up only when he asks or it is natural (we are shopping or talking about clothes or cleaning the closet/doing laundry) and then I tell him what I think and give him my reasoning exactly once unless he brings it up again. He is a smart man and he can take it from there. I've said my piece and that is where I leave it. I do give compliments and talk about things I do and don't like on other men freely.
Recently I've helped him pick out some linens he is really pleased with and has done a complete 180 on trusting me to buy things he'd like. I think he really sees that I respect his opinions and keep his priorities in mind while adding lots of "girl knowledge" to the arena. He has also started wearing his hair differently (which he did for his own reasons, but I like the style) and changed several of his grooming habits. He is in need of some new ts and pants and has decided on his own that he will buy a longer inseam and found some more subtle, adult graphic ts online that come in talls. I wouldn't want him to change himself for me, but he seems to be very happy with the changes.
I have a feeling that he will continue to ask my opinion and take me shopping as he gradually updates his wardrobe.
On the other hand, I appreciate that he has a good idea of what my personal style is and respects it. He thinks I look nice and encourages me to buy and wear things that make me happy, but also to stay true to myself. He also would like to see me dressed to the nines on occasion, but has offered to dress up himself in exchange, so no double standard.