fakruzaruret i found what you said to be very rude. Moreover, they actually mentioned that you could take the CSS codes yourself if you wanted. Also, from what I've seen, it's not only CSS; they've customized their site using extensions too. I think if you had asked for something specific instead of the entire CSS code, they might have been willing to help.
There are highly customized sites I really admire, like https://brickzip.com/, and if I wanted, I could obtain most of the customizations by following the mentioned methods. However, I don't do this without permission from the owners. Similarly, when I like the wording on special pages, such as “Forum Rules,” I contact the site owners and ask for permission, even though it would be simple to copy-paste.
On the contrary, when you use an open-source system, you should ask for permission to use it if the person openly shares the codes, rather than directly asking them for all their codes.
Similarly, just because something is open-source, you shouldn’t copy and further develop extensions to share without permission from the author.