My observations include ...
* Cultures bump against each other. Since humans have now filled most of the habitable space on the planet, this is inevitable for all practical purposes.
* When cultures meet, they rub off on each other. Usually this goes both ways. We get awesome things like chicken tikka tacos and Hawaiian-print dashikis. \o/
* Humans also have a nasty tendency to hurt each other. That doesn't seem to be going away soon either. :(
* No matter what you do or don't do, you can't please everyone, and it is unhealthy to try. Instead, examine yourself and set your standards of ethical behavior.
* People like to try out stuff. They dress up in costumes on Halloween, and some of those are rude, because that holiday is about breaking rules, which is a safety valve that society needs. People also explore new clothes, foods, hairstyles, etc. And all that stuff is what a culture is made of. We communicate who we are by the different details in our everyday lives. It is generally healthier to like new things than to dislike them. People who like new things are less likely to hurt each other than people who are reluctant and standoffish. Unless trying new things upsets someone else. That's awkward.
* Some useful questions to ask when exploring a thing that isn't part of your culture (or what someone else thinks should be your culture):
-- Did you get consent? Frex, if a person from that culture said you could use it or gave/sold it to you willingly, then you can definitely use it. If you took or copied it without asking, the matter is less clear. To make matters really interesting, some consent or even orders come from higher powers and pertain to people living in a certain area rather than belonging to a particular group; i.e. these are the instructions for inhabitants of this land. So then you have to consider whose consent you value more: the humans who are getting in your face about what you're doing, or the higher powers in charge of the place you live.
-- Does your use of something deprive someone else of using it? Objects are typically owned by one person or a small group. But music can be played by lots of people. Land is especially sensitive because it has often been stolen from its rightful owners, and some artifacts are the same. If they are special to the original culture, this can be very hurtful.
-- Does it send a specific message about an actionable trait? Some cultural things identify marital status, professions, or other important information. Tattoos in some culture are very nearly like medals. Certain garments may only be worn by holy people. It is generally a very bad idea to copy these exactly; it is not only rude but also misleading. However, it is often possible to create variations that have the same flavor but not the same problem. The Maori have created a style of artistic tattoo that resembles their cultural tattoos minus meaning. In the Southwest, sand paintings and kachina dolls are always made with a missing detail, precisely so they won't do anything but are now just nifty art.
-- Is this about your or someone else's body? This pertains to things like hairstyles and cosmetics. Each body belongs to the person(s) in it, period. You can do whatever you want with your body. But you have to let other people do whatever they want with theirs. By this rule, it is fine for white people to wear dreadlocks but not fine for them to forbid black people from wearing dreadlocks. White people wearing dreadlocks but banning black people from doing so is downright obnoxious.
-- What is the value of the thing you're using or doing? If it's worthwhile, then there's more reason to keep it up than if it's trivial. If you're making something new, that can be awesome. Or really awful. Figure out which.
-- Could some other thing be used instead of the thing that some people might not want you to have or do? If an equally good equivalent is available, it is often better to use that. If one is not available, you have more grounds to use or do this thing.
-- Did you pay for it in some way, or just grab it and run? Some things are bought and sold. Some are traded. Some are stolen. The less people get back for their stuff, the more likely they'll be upset.
-- Do you support the culture and its interests? There are other ways to give back than just paying for one item. You can support a culture's political aims, safety, right to make their own decisions, and so forth. That makes it less of a theft and more of a trade.
-- Do you know anyone from that culture? If you do, it's easier to see which things are safer to share and which are more likely to upset people. If not, you're probably flying blind. You can try looking it up, but the answers often differ.
-- What is the general relationship, if any, between the two cultures? Sometimes they like each other. \o/ Sometimes there isn't a close relationship. India and Mexico are nowhere near each other and not closely entangled, yet somebody thought up chicken tikka tacos anyhow. These are among the safest opportunities for sharing. In other cases, the cultures are fractious neighbors or one invaded the other. These are more fraught and likely to cause tension. That doesn't stop the crossover but does make people bitchy about it.
-- How much do you care about what other people think, and how much are you willing to talk about it? If you don't care, you can largely do as you please, but some of that will probably be rude. If you're open to discussing it, you've got some leeway and might actually get people to calm down some.