CSSketch
Plugin that adds CSS support to Sketch 3 for a faster design workflow
...The plugin detects stylesheet changes and reapplies them when the file is saved, creating a code-driven design workflow. Sass and Less support allows designers to use variables, functions, and other stylesheet conveniences. CSS processing is powered by the system WebKit engine, and stylesheet paths are stored inside Sketch documents for reuse. The project is no longer maintained and does not work with current versions of Sketch, but it remains an interesting example of programmable visual design tooling.