I think the current defaults are fine. I only ever use dark mode but some people use both. Respecting an existing preference is the way to go imo.
Compact is a preference, it can feel very cramped but some people have strong feelings about utilising screen space (honestly some non compact stuff is really silly, so I get it).
Vaporwave is neat if you like it but not generic enough for a default.
I think there is a good balance in the number of choices. I wouldn't add to it unless it's something distinct. I honestly wouldn't put time into making a site theme. You could do just a colour scheme but full themes are hard. I say this as a web developer based on my own experience. Also, from that, I've found that if users are happy, you're doing a really good job. Because if something is wrong or feels off you'll hear it fast.