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
I use it to manage all my color configs. it is based around the many base16 themes

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