Various css files for retro-theming. Each of these have been discussed here previously.

  - Windows 98: https://github.com/jdan/98.css
  - Windows XP: https://github.com/botoxparty/XP.css
  - Nes: https://github.com/nostalgic-css/NES.css
  - Mac OS (classic): https://github.com/sakofchit/system.css
  - DOS: https://github.com/vinibiavatti1/TuiCss

-counter strike 1.6: https://cs16.samke.me/?

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.

This is also featured in other Valve games. It's called vgui or something like that