I understand what "design systems" are trying to accomplish, as well as "CSS frameworks", and while I agree with the principles, and tout the advantages of them to others, I don't use them myself for my own projects. Or at least I don't use the systems and frameworks of others for the projects I work on.
The idea of a design system is to define a design language that can scale across the many different layouts and design challenges you might face and provides you a way to 'solve' these challenges in a consistent way that has integrity over the whole project. I do this, but I see it as my job to figure out and define what this means for each project, rather to approach a project with a "design system" in hand and apply it to the project. For me, the design system you come up with is kind of like what a "brand guide" is for graphic design. It's a great document to produce yourself, and then stick to.
As for CSS frameworks, that is kind of like one application of a design system. This is the design language translated into the CSS language, but you may also be using the same design system with other programming languages, or other apps (like Sketch or Illustrator) which don't use CSS, but would have 'frameworks' made out of other things that weren't CSS to accomplish the same task that the CSS framework provides for you in CSS. I think the best CSS framework is the one you create after solving a problem from scratch 4-5 times. Once you have solved the same problem a number of times, you finally understand how to abstract that nicely and write a helper (your framework) that ensures that every time you solve that challenge in the future, you are doing it in a consistent way.