CSS chapter - added explanations about CSS and fixed some language mistakes

Added some info about how CSS works (selectors - tags, classes and ids, declaration blocks...) with some useful links to w3schools pages. 
Changed some text that relates to my additions.
Fixed some language issues and typos. 
My first pull request!! :)
There are changes in the master branch that are clashing with yours :)

change mysite/blog/templates to blog/templates