Looks like they have their own stylesheet, not sure if they have it somewhere with documentation/etc.

I will say, BOOTSTRA.386[0] may be a good substitute; per it's name it's just bootstrap. I've actually used it in production[1] and it works just as one would expect.

[0] - https://github.com/kristopolous/BOOTSTRA.386

[1] - At the time it was just a gag prototype, and while I told everyone "if you don't like it feel free to switch it to normal bootstrap or one of your choice" but nobody bothered to do so since it wasn't a very complex UI and, frankly, it kept things clean.

When it comes to fonts there is this: https://int10h.org/oldschool-pc-fonts/fontlist/

It's the old IBM code page 437[1]. Lots of DOS applications were designed to look like this.

[1] https://en.wikipedia.org/wiki/Code_page_437

Reminds me of Turbo Pascal (5/6/7)

The site not capturing Alt + shortcut keys to control the menu is a big missed opportunity though.