Quark theme is the new default theme for Grav built with Spectre.css a lightweight, responsive and modern CSS framework. Spectre provides basic styles for typography, elements, and a responsive layout system that utilizes best practices and consistent language design.
You can find the source code for Quark at GitHub: Quark
You can find the source code for Spectre at GitHub: Spectre.css
Official Jekyll website: jekyll-organization