20. Adding a Shared Header & More State Management

02. Optional React Refresher 20. MainHeader.jsx: https://github.com/academind/react-complete-guide-code/blob/zz-reactjs-summary-updated/extra-files/MainHeader.jsx 20. MainHeader.module.css: https://github.com/academind/react-complete-guide-code/blob/zz-reactjs-summary-updated/extra-files/MainHeader.module.css