Tachyons was built for designing highly readable and 100% responsive interfaces with as little css as possible. Each module within tachyons is available on npm as standalone css that you can include within your own project. You can also consider just copy and pasting the source from here or from github.
Modules can be altered, extended, or changed to meet the design needs of your application. The most important parts are the mobile-first architecture and single purpose class philosophy. Class names and property values can be changed to fit the development needs of your team or the design needs of your application.