Where can I find i3/Sway themes, aside from the Sway Wiki?

people’s configs on github?

Any of the Unix porn communities on here or elsewhere, search for sway. Check the comments for dotfiles.

what kind of themes do you mean? just the client.* colors as documented in sway(5)

or also styling of terminal, gui apps and such?

more so in the latter case, maybe check out https://github.com/Misterio77/flavours based around base16 themes
I use it to manage all my color configs