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.
[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.
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.