Recs.
Updated
mini.css is a tiny CSS toolkit (under 7KB gzipped) that works well on most modern browsers and devices and allows you to customize it quickly and easily. It's written in Sass and tries to make use of HTML5 semantics and modern patterns to deal with common web design problems.
Specs
Pros
Pro Supports CSS custom properties (var)
No other framework supports CSS variables right now (as of November 2017). The latest alpha of mini.css supports this feature, making customization even easier.
Cons
Con Not widely used
A large community is always an advantage especially for open source projects. It means better documentation, continued development, and lowers the possibility for the project to be abandoned in the future since the probability for someone from the community to keep maintaining it is larger if the community is larger.