The Concise CSS framework is a completely free open source lightweight option for frontend developers. It’s written in Sass for the modern developer who can’t part from his/her favorite preprocessor.
If you visit the actual landing page you’ll realize the site is just as lightweight as the library.
Everything is bloat-free with a focus on ease of use. There are plenty of add-ons with the ability to code your own, too.
Concise.css is also available for free download on GitHub with a few example code snippets for setup. Feel free to download a copy and try it out in your next project.