The objective of these exercices is to become more confortable with css manipulation.

 Koan 1 : 

![graphical instructions](./css-koans-training.png)

 - add a css file that make header green and content red
 - use diplay flex to make content fill the entire page
 - add a 25% width left sidebar
 - add some uniform tabs on top of content, and a status bar on bottom 
