people’s configs on github?

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

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