Challenges

 - Implemented CSS for challenges (same as for Objectives, just different color)
 - removed redundant CSS for Objectives / Key Points
 - Clarified & rearranged challenge in lesson 02.
 - added contrib instructions.
Good - can you please put this in a separate PR from the changes to the lesson Markdown files so that it's easier for me to cherry-pick back into lesson-template? Or alternatively, make this PR directly against the lesson-template repo's `core` branch, and I'll then pull it into all the lesson repositories from there.