Self follow-up: https://macthemes.garden/themes/aa2e2f4e6f87-openstep-4/ and https://macthemes.garden/themes/168f21725acd-nextstep-4/ appear to actually have the correct assets for a NeXT look. Scott Naness seems to have been a cut above the rest for authenticity—there's also OS/2 Warp 4 and even vintage Windows 2.0 skins in his library.

Also, special mentions for obscure GUI clones:

- Xerox Star: https://macthemes.garden/themes/ede837fa5df1-xerox-star/

- QNX: https://macthemes.garden/themes/c46eae6cd818-mac-qnx/

- Solaris CDE: https://macthemes.garden/themes/8ba34a581676-macsolaris/

- the classic X Athena widgets: https://macthemes.garden/themes/533452549350-xlook-athenaxlo...

- Rhapsody, because obviously: https://macthemes.garden/themes/b0c635d1faf0-rhapsody-k2/