When I first discovered the prototyping capabilities for colors/themes/samples in the Chrome dev tools, my mind was blown away b/c of how fast I could iterate. A plug-in might take that to the next level. It's hard to say without me surveying more of what's out there and in use--I get the sense that at a certain level, teams rely more on tools like Sketch, inVision studio, Framer etc. for this.