Went searching and found a few more:
-PlayStation: https://github.com/micah5/PSone.css
-Gameboy: https://github.com/luttje/css-pokemon-gameboy
-Windows7: https://github.com/khang-nd/7.css
-Win95: https://github.com/AlexBSoft/win95.css
-Oldschool RPG: http://ronenness.github.io/RPGUI/
-MacOS 8: https://github.com/npjg/classic.css
-MacOS 6: https://github.com/sakofchit/system.css
And some that are more than just CSS:
-386 DOS: https://github.com/kristopolous/BOOTSTRA.386 (Bootstrap Theme)
-Terminal: https://github.com/xtermjs/xterm.js
-another Terminal: https://github.com/andersevenrud/retro-css-shell-demo
-90s website: http://code.divshot.com/geo-bootstrap/ (Bootstrap)
I had the 386 bootstrap theme as an option on a site I had several years ago. I did not like modals at the time. Make sure to test. Every other Bootstrap theme I tried at the time could simply drop in and work.