CSS is used to style a website and make it pretty. It is good practice while writing your HTML to think about repeated styles and good structure so you dont end up repeating the same blocks of code multiple times in your CSS. A bad plan not only leads to mountains of CSS but it will also make it near impossible to makes any updates to the website later on. Just learn the basics of HTML and CSS now and in later workshops we will look at techniques to building your website layout and styling.
Similar to the HTML notes, I had created my own CSS notes for the workshop. I still think all the CSS tutorials on w3cSchool are better than anything I could create. You can find the w3cSchool CSS tutorials at:
Once you get through that tutorial give this one on CSS layouts a try: