To contribute, fork and clone the repo. Create a branch with the same of the issue that you are working on, and do a pull request after you have completed the work.
git checkout -b No2-UpdateColorOfNavBar
Make your changes
git add Navbar.js
git commit -am 'changed colors in navbar'
git push -u origin No2-UpdateColorOfNavBar
but use an editor to simplify the process.
Please try to allow one commit message per file so others can follow along with your work.