So, the class itself isn't designed to teach CSS, but it is designed for people to learn CSS as they go along. The HTML and CSS overview; Examining with browser developer tools class will be the explanatory bit you're looking for--it will do a brief explanatory bit, show you a lot of good resources and references to learn CSS with, and show you a bit about how to use the developer tools of your browser to see what's going on behind the scene.

Additionally, the Tabulua Rasa Plain style is designed to take care of some of the more finicky parts of layouts like where the columns go, allowing you to focus on the simpler parts of CSS customization to get your feet wet with. This will be explained in Custom CSS; The formation of a Style and Using the Advanced Customization Area.

Then, the layout making series of classes will give you opportunities to ask questions and get assistance about CSS things you don't understand.