Yes, this is true that we lean heavily into Tailwind CSS by default. However, when I implemented our "new" component system in 2021, I designed it so that there was a path forward for Bullet Train on vanilla CSS or Bootstrap if anyone wanted to implement it. I don't have any plans to do it myself, but people (including Tamik, the original theme author for Bullet Train) have expressed an interest. I'd love to see it happen. You can get a sense for how this would be possible from our theme docs at https://bullettrain.co/docs/themes.