Adding some style tweaks and a different CSS reference link to Class 2

- Highlighting the letters in Cascading Style Sheets (CSS) to show the students more clearly where the acronym comes from.
- Changing the CSS reference link to the Mozilla Developers Network CSS reference page. The original link to W3C schools has broken links for the selector and property detail pages (e.g. http://w3schools.com/css/pr_background-color.asp).