I’ve found the Arc themes work quite well with Cinnamon, and they’re in the official repositories, and if you use Kvantum to theme qt apps Arc and Arc-Dark are in there by default, and it’s even in Flathub in case you use Flatpaks. It’s not an exciting theme, but it’s the easiest non-Adwaita or Breeze theme to get consistent theming across different apps from different sources with, which I personally like a lot. The icon theme kinda sucks though, at least in my opinion, so I’d recommend something like Papyrus instead of the default Arc icon theme.

Adapta and Adapta-Nokto also work quite well from my experience. Most of the really popular themes that work in Cinnamon make pretty heavy use of transparency though, and that’s not really my style, so hopefully someone else can fill that in for you. I know Numix is a big one, but that’s about it.

Also, if you’re looking for backgrounds on Arch there is always the official archlinux-wallpaper package. Some of those are kinda neat, and it’s easy to never realize they exist.