Primer CSS is a free open-source CSS framework that is built with the GitHub design system to provide support to the broad spectrum of Github websites. It creates the foundation of the basic style elements such as spacing, typography, and color. 

This systematic method makes sure our patterns are steady and interoperable with every other. Its approach to CSS is influenced by Object-Oriented CSS principles, functional CSS, and BEM architecture. It is a highly reusable model.

CSS Flexbox and Responsiveness: https://www.geeksforgeeks.org/primer-css-flexbox-responsive-flex-utilities/