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
would wstunnel help? just run that between both machines and pick whatever works best, even if that is ssh